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/.v

THE

ELEMENTS OF EMBRYOLOGY.

^ ^ d

FELIX)

e'RMhECTOR IN PHlaluiAMvi ^, .

Fester ^ft- Altchn <• /

> AXn TinP. T.ITK f

AMD THE LATE

FRANCIS M. BALFOUR, M.A., LL.D., F.RS.,

rSLLOW OF TBINITT COLLEGE, CAMBBIDOB,
AMD PB0FB880B OF ANIMAL MOBPHOLOOT IN THE UNIVEBfllTY.

SECOND EDITION REVISED.

EDITED BY
ADAM SEDGWICK, M.A.,

FELLOW AMD ASSISTANT LECTUBEB OF TBIKITT COLLEGE, CAMBBIDGE,

AMD

WALTER HEAPE,

DEMONSTBATOB IN THE MOBPHOLOGICAL LABOBATOBT OF THE UMITEBBITT

OF CAMBBIDGE.

'Eonbon :

MACMILLAN AND CO.

1883

{The Right of Translation it reserved,]

■ •

538072

FBrar«D BT C. 1. CL4I. B,A. k BC

PREFACE TO THE SECOND EDITION.

When this little work first appeared, it was put for-
ward as a Part I, to be followed by other Parts. That
plan was however soon abandoned. Nevertheless the
volume seemed to have a place of its own ; and my dear
lost friend undertook to prepare a second edition, in-
tending to add some account of the development of
the Mammal with a view of making the work an
elementary introduction to vertebrate embryology more
particularly suited for medical students. He was occu-
pied with the task at the time of his sad death; and
indeed a melancholy interest is attached to some of the
sheets, by the fact that he had taken them to Switzer-
land with him, on that fatal journey.

All the first part up to p. 160 he had passed for
press ; and he had further revised up to about p. 202.
The whole of the rest of the volume has been under-

vi PREFACE.

taken by Mr Adam Sedgwick and Mr Walter Heape.
They have attempted to carry out as far as possible
what we believe to have been Balfour s views, and
trust that the public will judge leniently of their
efforts to perform a difficult task. I have myself been
able to do no more than offer general advice from time
to time; and though it has not been thought advisable
to change the title, the merits as well as the responsi-
bilities of the latter part of the work must rest with
them.

M. FOSTER.

Trinitt Colleob,
Caxbbxdoe,
March, 1883.

TABLE OF CONTENTS.

PART I. THE fflSTORY OF THE CHICK.

CHAPTER L
Thx Stsuctubb of TBS Hen's Ego, and tbx Ohahoss wmoH tamm

PLACB UP TO TEX BlOINNINa OT InGUBITIOR . pp. I — 14.

The shell and sheU-memhrane, i — 3. The alhomen, 8. The
vitelline memhrane, 4. The yolk, 5 — 7. Area opaoa, 7. Area
pellaoida, 8. The straotnze of the blastodenn, 7— la Beoapita-
lation, 10. TheoTarian OTorn, 11 — 15. The descent of the oynm
along the ovidnct, 15 — 17. Impregnation, 17. Segmentation,
18 — 24.

CHAPTER II.

BbOV SnmiABT of TBM WHOLB HuTOBT of iNOUBinON,

pp. ^5— 47-

The embryo is formed in the area peUndda, 95. The germinal
layers, 35, 96. The extensbn of the blastoderm over the yolk, 96.
The vasenlar area, 97. The head-fold, 97 — 36. The tail-fold, 37.
The lateral folds, 37. The yolk-sao, 37. The alimentary canal, 39.
The nenxaltnbe, 39, 40. The body-cavity, 41. The somatopleure, 41.
The splanchnopleore, 49. The stalk of the yolk-sao, 49, 43. The
amnion, 43 — 46. The allantois, 46, 47.

TABLE OF CONTEKTS.

CHAPTER III.

Thk CnisaxB which taik puce noKraa the Fibbt Day of Imcf-

BiTlOM ... pp. 48—76.

VBTiationB in the progreaB of dcvalopaient, 4S, 49. Tlio embryonio
shield, 49. FonmitioD of hypoblast, ji. The gernunal wall, 51.
Tha piimitiTe atreak, ji — 54. Formation of primitivs streak meao-
bl&Bt. 54, 55. Hjpoblastdc meaablast, 55. PrinutivQ groove, j6, 57.
The nolocborf, ^9—61. Tlie mednUnrj groove, 61, 6j. Amnion, 63.
The changeB taking pluce in (ho throe layeni, 63 — 66. The germinal
wall, 6i, 66. Tho iocrease of the head-fold, 66. The closare of the
raedallMy canaJ, 66, 67. The cleavage of the meeoblaat ; fonnation
of Bpuichnopleare and Bomatopleiuo, 68. The vertebral and lateral
plateB,6g. The meaublastiu Eomites, 70. The sinnB thomboidollB, 71.
The Beurcntoric paBBage, 71^74. Formation of the vascular area.
74, 75, Becapitulation, 75, 76.

CHAPTER IV.

Thk CaiKOKS wnicn take pLiOE dcbiso the Fhibt Half if tee
Secokb Dat ... pp. 77 — 96.
IndewEing distinatnosB and prominonoe of embryo, 77. The first
cerebral yeaicle, 78, 79. Tho auditory pita, 8i. Increoae in number
of mesoblaalio aomJteB, 81. The fore-gut, 8j. The heart. 81—89.
The vaHOular BjBtam, 89—94, Forroation of blood-YBBselB, 95—94-
Tho mditneat of tlio Wolffian duet, 94. Summary, 94, 95.

ThB CBASQEB which take PI.AOE DCBIBO THB SECOND HAU OJ" THE

Hecomd Day . . . pp. 96— loH.
Increaaing pcominence of the embryo; the tail-fold and lateral
folds, 06. Continiwd oloBure of meilaUary canal, 96—98. Tlir
brain, 98 — 101. The optio veaido9, 98. The second aud third oerebral
veBiclet!, loo. Tho cerebral hemidphercs, 100. First appearance of
cranial nervea. 100, toi. The notochord. loi. The cranial flemre,
loi. The auditory vesicle, 101. Increase of curvature of heart, 101.

TABLE OF CONTENTS. IX

I03. AnrionlAr appendftges, loi. Vatcular Syttem, 104—106.
Commenoement of oirooUtion, loa. The primitive aorUs and first
pair of aortio arohee, loa, 103. The vitelline Teuela and einui ter-
minalis, 103, 104. The course of the cironlation, 105. The seoond
and third pairs of aortic arches, 105, 106. The Wolffian dnot and
first aiqpearanoe of Wolffian body, 106. The growth of the amnion,
107. The first appearance of the allantois, 107. Summary, 107,
108.

CHAPTER VI.

Tbs Chanqes wricb take PLiCB ouBiira tbk Tninn Day,

pp. 109 — 194.

The diminution of the albumen, 109. The spreading of the opaque
and vascular areas, 109, no. The vascular area, no— 113. The
continued folding-in of the embryo, 113. The increase of the amnion,
1 1 3. The change in position of the embryo, 1 1 3— 1 16. The curvature
of the body, 116. The cranial flexure, 116, 117. The brain, 117 — 133.
Growth of the vesicle of the cerebral hemispheres, 117. The lateral
ventricles, 117. The vesicle of the 3rd ventricle or thalamonocphalon,
117. The rudiment of the pineal gland, 117, r 18. The infundibulum,
1 1 9. The stomodaeum, 1 1 9. The pituitary body, 1 1 9 — i a i . Changes
in the mid-brain, the corpora bigemlna, crura cerebri and iter, m.
Changes in the hind-brain, the medulla, cerebellum, 4th ventricle,
m, 133. Changes in the neural canal, na, 133. The cranial
and tpinal nerves, 123 — 133. The neural band, 133—136. The fifth,
seventh, ninth and tenth cranial nerves, 136, 137. Later develop-
ment of cranial nerves, 137 — 139. The spinal nerves, 139. The
shifting of point of attachment of nerves, 131. Anterior roots, 131.
The eye, 137 — 156. The first changes in the optic vesicles, 133, 133.
The secondary optic vesicle and development of the lens, 134—137.
The choroidal fissure, 137 — 140. The choroid, sclerotic and cornea,
140, 141. The further development of the optic vesicle, 141. The
ora serrata, 143. The iris, 143. Pigment epithelium of choroid, 143.
The dliaxy processes, uvea, ciliary muscle and ligamentum pectinatum,
144. The histological changes in the retina, 144 — 146. Optic nerve,
146, 147. The choroid fissure, 147. The pecten, 148. The histo-
logical changes m the lens, 149, 150. The vitreous humour, 150.
The cornea, 150—153. The aqueous humour, 153. Summary of the
development of the eye, 154, 155. The eyelids, 155. The lacrymal
glanda and duct, 155, 156. The organ 0/ hearing, 1^6 — 161. Closure

TABLE OF CONTENTS.

of til 0 iiuilitoiy involution, :57. The otio veeide, is;. T!ie mem.
branouB labyrinth, 158, i$g. The oaaBona Iftbyrintli, 159, 160. Com-
jioriBon of ear nitli eye, lAo, 161. The organ of emeU, 161, 163.
The olfactory lobea and nerves, 161. The viscfral arehet and vUccral
clffu, 16] — ili;. Superior maiillar;, and fronto- nasal prooesBeii,
164, 165. Fate of first vi«ersl cleft, 165, 166. Tho msatua aodi-
toriua eiternuB, 166. The tympanic membrttue, 166. The EuBtttchian
Inbo Bud tympanic cavity, i6f, iGC. The fenestra oTalis and rotunda,
166. Tho columella, 166, 167, The naicular tytltm, 167—170. The
ooitic arches, 1G7. Changes in tba heart, 167, i6g. The venouB
HyBl«m, idg, 170. The meatus venoBus, cardinal veina and ductus
Onvieri, ifig, 170. The alimentary caiuil, 171 — 185. Folding in of
the splanchnopleure, tail-fold, 171, 171. The mesentery, 173, 173.
(Esophagus and stomach, 173. The intestine, 174. The postanal
gut, nenrenterio canal and prootodiemD, 174 — 176. The lungs,
176 — 178. The liver, 178 — iS[. The pancreas, 181. The thyroid
body, :8:, iSi. Thaspleen, iSi. The growth and blood-vesselBoI the
allantoia, 181—184. ^''< ^"lotlaet, 185—193. The muscle-plates,
iSG — 189. The intermediate cell-mass and Wolffian body, 189 — 193.
A. typical Wolffian tubule, 193. Change of position of Wolffian duct,
193. Sommaiy, 193, 194.

CHAPTER VII.
The Ohanoes which taxi placb on the Focbtb Dit, pp. 195 — 131.
Appearance on opening the egg, 195. Growth of amnion, 195, 196.
The vitelline duct, 196. Increase of cranial fleiure and tail-fold, 19G —
198. The first appearance of the limbs, 198. The growth ot the
brain. 100. Tho face, 101. Chaages in the naegJ pits, loi. The sto-
modnnm and month, goi, 103. The cranial nerves, 303. Changa in
the m£tohUulie tomiui, 504 — iii. The roembranouH vertebral
column, 10;,. The sooondary segmentatian of the vertebral column
and formation of the permanent vortehra;, 305-107. Becapitnlotion,
107. 3oS. The changes in the notochord, 108—911. Ossification of
vcrlebne. 109, 110. Tho changes in the muscle plates, iti, itl.
Wolffian body and duct, 111-114. The MUHeriaa duct, 514—118.
The kidney and ureter, ii3— 110. The ovaries and tastes, iia—tti.
Fate of the embryonic urinogenital organs, «i3, 114. Changeg in tlu
arterial syslum, 114 — ii6. Changes in the venous system ; veins of
the liver, tiG — 119. Changes in the heart ; Iha ventricular septum,
ill), 330. Snmmory, 730, 131.

TABLE OF CONTENT& XI

CHAPTER VIIL

The Changss which tamm PLios ox thi Futh Day, pp. 133—974.

Appearance on opening the egg, 931. The changefl in the limbii,
^33* ^34* ^^ pectoral and pelvic girdles ; the ribs and itemnm, 934,
335. The development 0/ the skull^ ^35— ^46. The craniom, 935.
The parachordals and notochord, 937, 338. The inhecvda, 339 — 141.
The sense capsules, 341, 941. Membrane and cartilage bones, 349.
Skeleton of Tisceral arches, 341 — 345. Table of bones, 346. The
changes in the face, 346 — 351. The posterior nares, 351. Changes in
the spinal cord ; its histological differentiation, 35 1 — 354. The central
canal; and the posterior and anterior fissures, 354 — 356. Clianges
in the heart, 356—364. Septum in the bulbns and semUunar valves,
357 — 359. The cardiac valves, 363. The foramen ovale and Eustachian
▼alve, 363 — 364. The pericardial and pleural cavities, 364 — 369.
Histological differentiation and the fate of the three primary lajers,
369 — 373. Summary, 373, 374.

CHAPTER IX.

Fbox the Sdeth Day to the £n> of Ixcubatioe, pp. 375—303.

The appearance of distinct avian characters, 375. The fcetal
appendages during the 6th and 7th days, 376 — 378. During the
8th, 9th and loth days, 178. From the i ith to the i6th days, 378, 379.
From the i6th day onwards, 179, 180. Changes in the general form
of the embryo during the 6th and 7th days, 380 — 383. During the
8th — loth days, 383. From the i ith day onwards, 383. Feathers, 383.
Nails, 383. Ossificadon, 183. Changes in the venous system before
and after the commencement of pulmonary respiration, 383 — 389.
Changes in the arterial system, the modification of the aortic arches,
189 — 397. Summary of the chief phases of the circulation, 397—303.
HatrJring, 303.

TABLE OF CONTENTS.

THE HISTORY OF THE MAMMA-
LIAN EMBRYO.

ISTBOBDCTWS, pp. 307, 308.

Oenebiil Develofitent of tos £kbbto

pp. 305—341.

The oVBriou ovam. jog. 310. The egg-mcinbrimtis, 31a. Ma-
turation nnd impregnBtioQ, 310 — 31]. Segmentation 1 311^-314- The
blaBtoduimio veaiclo, 314 — 316. The fonnation of the laycta, 316 —
310. The primitive etreok and groove, 319, 310. The medullary
grooTB, 3)0, 331. The mesoblaBt, 311—315. Tho notoehonl, 315, 316.
The radiment of the neurcaterio canal, 316. Iteoapitulntioll, 3)6.
The vascular area. 316. General growth of the embiyo, 317^334.
The human embryo, 335—341. Embryoa of guioea-pig, etc. with so-
il of thelayera, 341.

CHArTER XI.

Ehbhtonio Meubrines a

I Tole-Sac .

pp. 34>— 364-

The typical dtvctopment of the fmbryonie mfinbrano, 34J — 351.
Vascular area of rabbit, 343 — 346. The ;aU(-sao or ambitical veaiole;
amnion, 343. The aubional merabrono, 346. Attachment of blaato-
dermio vesiclH to otoriaa wallfl. 347. The formation of the ohorion,
34R, Mesoblast and blood- snpplj' of the allantoia, 34S, 349. The
placenta, 349. 350. The fate of the embryonic membranes, 350 — 351.
PecLdoate and non-decUuate tjpe of placenta, 351. ComparatiFe
hUtorg of Ihi matnraaiian falat membraiia, 357—359. Fcetal mem-
branes of MoDotremata and Maraiipialia, 351. The diBOoidol pla-
centa, 353, 354. The metadincoidal placenta, 354 — 353. The zonary
placenta, 358, 359. The diffusa and polycotyledonary placenta, 359.
Comparative histologj of the placenta, 359 — 363. Evolution at the
placenta, 364.

TABIX OF OONTEariB. XUl

CHAFTEBXIL

Thb Devsxx>pmsht cur thb Oboaxs is IfAWWii.Ti, pp. 365—433.

Tk« orgatu derived firom the eptUattf 365^400. Hain, 365.
GlmndB, 366. The hind-bimizi, 367 — ^37a The mid-bnun, 370, 371.
General derelopment of Ibie-liram, 371. ThaUmeiioeplialon, 371
—376. Pitniteiy bo^, 373, 373. Pineal gland, 373— 37<!>- Ceiefcial
hemisi^ieree, 376 — ^385. The oUaetoiy lobes, 385. HUto^enesifl oi
brain, 385 — 387. The eyee, 387 — ^390. Choroid alii, memfarana
eapsolo-papillaiifl and arteria eentxalis letuuB, 389. The aoditoiy
organ, 390^397. AeeeBsoiy aoditoiy stroetarea, 397 — 399. Thenaaal
organ, and organ of Jaoobeon, 399. Cranial and spinal nenres ; sjm-
pathetie system, 400.

Organs derived from the wkemMoMt^ 400—417. The Tertebral
oolmnn, 400, 401. The sknll, 401. The Tisceral arches, 403. Man-
dibular and hjoid arches; malleos, incus, and stapes, 403 — 405.
Bibs; Btemom; pectoral and pelvic girdles, 405. Skeleton of the
limbs, 406. Body-cayitj; pericardial, pleoral cavities and dia-
phragm, 406.

The vmevlar tyttem, 406—413. The heart, 406, 407. The ar-
terial system, 407 — ^409. The venoos system, 409 — 413. The supra-
renal bodies, 413, 414. The nrinogenital organs, 414—417.
Wolffian duct and body ; kidney ; ovary and testis, 414, 415. Genital
cord, 415. Urinogenital sinus and external generative organs, 415
— 4«7-

AUmerUary canal and its appendages, 417—423. Splanchnic
mesoblast and mesentery, 419, 410. Stomodsum, 420, 431. Hard
and soft palate, 430, 43?. Teeth, 431. Proctodsum, 423.

APPENDIX . . pp. 423-471.

Incubators, 4«3— 4«5. Hardening reagents, 435—418. Staining
reagents, 438—433. Imbedding, 433—434. Cutting sections, 434,
435. Mounting sections, 436. Preservation of embryos as a whole*
436, 437. *

XIV

TABLE OF CONTENTS.

Practical directions for obtaining and studying cTUck embryoi, 437
— 460. Examination of a 36 — 48 hoars embiyo, 437 — 444. Of an
embryo of about 48 — 50 hoars, 444 — 447. Of an embryo at the end
of the 3rd day, 447 — 451. Of an embryo of the 4th day, 451^453-
Of a blastoderm of 10 hoars, 453 — 456. Of an onincubated blasto-
derm, 457. Of the process of segmentation, 458. Of the later changes
of the embryo, 459. Of the development of blood-vessels, 459, 460.

Practical directions for obtaining and studying Mammalian em-
hryoSf 460 — 471. Animals and breeding, 460, 461. Examination and
treatment of segmenting ova, 461 — 464. Of the blastodermic vesicle,
79 — 90 hoars, 465. Of the blastodermic vesicle of 7 days, 465, 466.
Of an 8 days embr3ro, 466—468. Of an embryo of 8 days is hoars,
4681 469. Of the fcetal membranes of an embryo of 14 days, 469,
470.

NoTl A. Automatic microtome, 471.

NoTl B. New method of mounting sections^ 471.

PART I.

THE HISTORY OF THE CHICK.

CHAPTER L

THE STRUCTUBE OF THE HEN'S EQG, AND THE CHANQES
WHICH TAKE PLACE UP TO THE BEGINNINa OF IN-
CX7BATI0N.

In a hen's egg quite newly laid we meet with the
following structures. Most external is the shell (Fig.
1, 8.), composed of an organic basis, impregnated with
calcic salts. It is sufficiently porous to allow of the
interchange of gases between its interior and the exter-
nal air, and thus the chemical processes of respiration,
feeble at first, but gradually increasing in intensity, are
carried on during the whole period of incubation.

It is formed of two layers, both of which may contain
pigment The inner layer is by fSeu: the thickest, and is
perforated by vertical canals which open freely on its
inner aspect. Superficially these canals appear to be
closed by the extremely thin outer layer. They are
probably of some importance in facilitating the pene-
tration of air through the shell.

Lining the shell, is the shell-menibrane, which is
double, being made up of two layers : an outer thicker

F^^ B. 1

THE hen's egg. [chap.

(Fig. 1, 8. m.), and an inner thinner one (i. s. m.). Both
of these layers consist of several laminaB of felted fibres
of various sizes, intermediate in nature bettreen connec-
tive and elastic fibres.

DiAQRAiouTio Sectio.v OF AS Untncubated Fowl's Eaa
(modified from Allen Thomson).

bl. blastoderm. u>. y, wliite jolk. This conaiets of a central'
flask-shaped caaas ild<1 a number of hiyers arranged ixat-
centrically aroiuid this. y. ff. jellow jolk. v. t. vitellino
membrane. *. layer of more Suid albumen immediately
eiirrounding the yolk. u>. albumen cousiatiiig of alternato
denser and more fluid layers, eh. I. clialaza. a. eh. air-
chamber at the brood end of the egg. This chamber IB
merely a space kft between the tno layers of the shell-mem-
brane. I. ». m. internal layer of ■siiell-membraao, *, m,
external layer of shell-membrane. (. sfaelt.

l] the white of the eqg. 3

Over the greater part of the egg the two layers of
the shell-membrane remain permanently in close appo-
rtion ; but at the broad end they tend to separate, and
thus to develope between them a space into which air
finds its way. This air-chamber, as it is called, is not
to be found in perfectly fresh t?ggs, but makes its
appearance in eggs which have been kept for some
time, whether incubated or not, and gradually increases
in 8126, as the white of the egg shrinks in bulk from
evaporation.

Immediately beneath the shell-membrane is the
white of Oie egg or albumen (Fig. 1, w), which is, chemi-
c&Uy speaking, a mixture of various forms of proteid
material, with fatty, extractive, and saline bodies. The
outer part of the white, especially in eggs which are not
perfectly fresh, is more fluid than that nearer the yolk.

\ia avera^ compoaition nmj be taken as
IS'O p. c. proteid matter,
Vb p. c. fat and extractives,
'S p. o. saline matter, chiefly aodlc and potsaaia chlorides,
with phosphates and sulphates,
86-0 p. c. wat«r.

Tli6 white of the egg when boiled shewa in section alter-
nate concentric layers of a transparent and of a finely granular
opaque raaterial. In the natural condition, the layers corre-
■jtooding to these opaque layers are composed of more fluid
albumen, while those corresponding to the transparent layers
AM lesB fluid, and consist of networbi of £bras, containing fluid
tD their nuebes. The innennoat layer, however, immediately
Btmounding the yolk (Fig. 1, x.), ia of the more fiuid finely
gnraular kind.

In e^B which have been hardened a spiral arrange-
t oi the white may be observed, and it is possible to
1—^

THE hek's Eoa. [cha**-

tear o£f laminEe in a spiral direction from left to rigbt»
from the broad to the narrow end of the egg.

Two twisted cords called the chalazm (Fig. 1, ch. t)»
composed of coiled membranous layers of denser albU"
men, run from the two extremities of the egg to tb^
Opposite portions of the yolk. Their inner extremiti«*
expand and merge into a layer of denser albumen bu^"
rounding the fluid layer nest the yolk. Their outeJ"
extremities are free, and do not quite reach the onte^
layer of the white. Thus they cannot serve to suspeB^l
the yolk, although they may help to keep it in position*;
by acting aa elastic pads. The interior of each chaJaott
presents the appearance of a succession of opaque white
knots ; hence the name chalazEe (hailstones).

The yolk is enclosed in the vitelline membrane (Fig.
1, ti. t), a transparent somewhat elastic membrane easily
thrown into creases and wrinkles. It might almost be
called structureless, hut under a high power a fine '
fibrillation is visible, and a transverse section has a
dotted or punctuated appearance ; it is probably there-
fore composed of fibrils. Its affinities are with elastic
connective tissue.

The whole space within the vitelline membrane is
occupied by the yolk. To the naked eye this appears
tolerably uniform throughout, except at one particular
point of its surface, at which may be seen, lying imme-
diately under the vitelline membrane, a small white
disc, about 4 mm. in diameter. This is the blastoderm,
r dcatricula.

A tolerably typical cicatricula in a fecundated egg
will shew an outer white rim of some little breadth, and
vithin that a circular transparent area, in the centre of

THE WHITE YOLK. 6

again, there is an opacity, varying In appearance,

letimes homogeneous, and sometimes dotted.

The disc is aiways found to be uppermost whatever

the position of the egg, provided there is no restraint

the rotation of the yolk. The explanation of this is

be sought for in the lighter specific gravity of that

portion of the yolk which is in the neighbourhood of the

^, and the phenomenon is not in any way due to the

Vtioa of the chalazse.

A section of the yolk of a hard-boiled egg will show

it it is not perfectly uniform throughout, but that

flwK is a portion of it having the form of a. flask, with

funnel-shaped neck, which, when the egg is boiled,

does not become so solid aa the rest of the yolk, but

Kttajns more or less fluid.

The expanded neck of this flask-shaped space is
immediately underneath the disc, while its
bulbous enlargement is about in the middle of the yolk.
We shall return to it directly.

The great mass of the yolk is composed of what is

wwn as the yellow yolk (Fig. 1, y. y.). This consists

spheres (Fig. 2, A.) of from 25/* to 100/*' in diameter

led with numerous minute highly refractive granules ;

spheres are very delicate and easily destroyed by

When boiled or otherwise hardened in situ,

IBJ assume a polyhedral form, from mutual pressure,

granules they contain seem to be of an albuminous

aa they are insoluble in ether or alcohol.

Qtemicall; speukiag tbu ;olk is characterized by the presence

laige quiUitities of a pruteid matter, having miLDj affinities

^bulin, and called viuUia. This exists in [wculiar

' ;* = -00I mm.

oitiea
jociu-

6 THE hex's egg. [CHAP-

tion with the remarkable body Lecithin. (Compare Hoppe-
Seyler, Bdb. Phy». Chem. Atud.) Other fatty bodies colotirmg
matters, extractives (and, according to Dareste, starch m smatt
quantities), &c are also present Miescher (Hoppe-S«yler,
Chem. Unttrmch. p. 502) states that a considerable qnantityot
nudeCn may be obtained fix)m the yolk, probably from t»
spherules of the white yolk.

Fio. 2.

o ®

,i, WUow yolk-8i^l.ere filled with fine granules. The outline of

th^ »l>hMv has been rendered too bold.
it >Yhit^ \»lk««phore* suui spherules of various sizes and pre-

•ewtiivj: diilexvixi apwAwuio^eBSw ^It is Terr difficult in a

Y<\\\Wv,t tv^ i:i\e a sA^i&fActoTT ^presentation of these pe-

«^u^r ^ir^^'turw.^

Vhx^ \vy.o\^ wVk ::v,i;5i fs^mun^ the cr&at mass of the
ouuw \o.\V ^^ ^^':^.^\i c\:v^rr4Al> bv a thin laver of a

hW >\^v ,s: , : s- Nv^kv;cr*,v, xv^s^-^ ;iaicr=iC3kih the disc,
^^^ \\v.v, .^^ »> v\,^ Ai li,^^ :!CVm tVirs::.5L jkS iT were, a
V».\> ^^5A \\ '^ s .^ : % V N^vt w^^^TSi T^sftSv ixi^ii^iia^elT under

^r vvVv*-»-vvvN V K >^\^» it ^v4v4i^ « itof loudened

I.] THE YELLOW YOLK, 7

white yolk is generally known as the "nucleus of
Pander."

Concentric to the outer enveloping layer of white
yolk there are within the yolk other inner layers of the
same substance, which cause sections of the hardened
yolk to appear to be composed of alternate concentric
thicker laminae of darker (yellow) yolk, and thinner
laminss of lighter (white) yolk (Fig. 1, w, y.).

The microscopical characters of the white yolk
elements are very dififerent frAn those of the yellow
yolk. It is composed of vesicles (Fig. 2, jB.) for the most
part smaller than those of the yellow yolk (4/Lt — 75/i),
with a highly refractive body, often as small as 1/i, in
the interior of each ; and also of larger spheres, each of
which contains a number of spherules, similar to the
smaller spheres.

Another feature of the white yolk, according to His,
is that in the region of the blastoderm it contains
numerous large vacuoles filled with fluid; they are
suflSciently large to be seen with the naked eye, but do
not seem to be present in the ripe ovarian ovum.

It is now necessary to return to the blastoderm. In
this, as we have already said, the naked eye can distin-
guish an opaque white rim surrounding a more trans-
parent central area, in the middle of which again is a
white spot of variable appearance. In an unfecundated
cicatricula the white disc is simply marked witii a
number of irregular clear spaces, there being no proper
division into a transparent centre and an opaque rim.

The opaque rim is the commencement of what we
shall henceforward speak of as the area opaca; the
central transparent portion is in the same way the

8 THE hen's egg. [chap.

beginning of the area pelludda. In the part coire-
sponding to the area opaca the blastoderm rests imme-
diately on the white yolk ; underneath the area pellu-
cida is a shallow space containing a nearly clear fluid,
to the presence of which the central transparency seenu
to be due. The white spot in the middle of the area
pellucida appears to be the nucleus of Pander shining
through. I

Vertical sections of the blastoderm shew that it ia
formed of two layers. The upper of these two layers
is composed, see Fig. 3, ep, of a single row of cell^
with their long axes arranged vertically, adhenngl
together so as to form a distinct membrane, the edge o^
which rests upon the white yolk. After staining witU
silver nitrate, this membrane viewed &om above shews)
a mosaic of uniform polygonal cells.

Each cell is composed of graanlar protoplasm filled
with highly re&active globules ; and in each an oval ou-;
cleus may be distinguished. They are of a nearly unifona'
size (about Q/*) <*^'^'' ^^^ opaque and the pellucid areas.

The under layer (Fig. 3, /). 's composed of cdlsj
which vary considerably in diameter; but ev«tt the'
smaller cell^ of this layer are larger than the cells of th«
upper layer. They are spbeiical, and so filled with
graDtiles and highly n^&active globules, that a nudeus
can raivly be seen iu them: in the larger c^Is these
globules are identical with the smaller white joUb
qiherea. |

Tittt cells of this lajraar do ikot form a distiDct meauT^
bnae like tlie cdts of the upper layrr, but lie as a;
■DIMirtiat iiTUipiIar nMwurk of cells bvtwtyn the uppes;
li^er Mu) the bed of while \>^k ua whk^ th«

:^ th« blastotieaiL

I-l THE BLASTODERM.

resta. The lowest are generally the
largest, The layer is thicker at the peri-
phery than at the centre : and rests ol
■ l^ of white yolk, from which it is in
parts separated by a more or less de-
veloped cavity, containing probably fluid
yolk matter about to be absorbed. In
the bed of white yolk nuclei axe present,
which are destiaed to become the nuclei
of cella about to join the lower layer of
tho blastoderm. These nuclei are gene-
*»IIj more numerous in the neighbour-
hood of the thickened periphery of the
Wulodenn than elsewhere. Amongst
lie lower layer cells are to be found

Fio. 3. -y ■

ms or A BuLSToDEiui of a Fowl's Esq

,T TttB COMMBSCEMENT OF IsCUBATION.

! thin but complete upper layer ep com-
posed of coluniitar ceils rests on the in-
complete lower layer I, composed of larger
and more granular cells. The lower layer
» fllioker in some places thao in othera,

ipodally thick at the periphery.

below the uuder layer marks tlie
of the white jolk. The larger

formative cells are seen at b,
lying OQ the whit« yolk. The figure does
not take in quite the whole breaiith of the
bloslCKlerm ; but the reader must under-
stand that both to the right banfl ani! the
left ep is continued brther than /, so that
at the extreme edge it rests directly on
th« white yolk.

peculiar large Bpherical bodies, wliich superficially i&'
aemble the larger cells around them, and have beei*
called formative cells. Their real nature is still vety
doubtful, and though some are no doubt tnie cells,
iitliere are perhaps only nutritive roasses of yolk.

The opacity of the peripheral part of the blastoderm
is in a large measure due to the collection of the lower
layer cells in this region, and the thickening, so caused,
appears to be more pronounced for a small arc which
subsequently constitutes the hinder border of the area
pellucid a.

Over nearly the whole of the blastoderm the upper
layer rests on the under layer. At the circumference
however the upper layer stretches for a short distance
beyond the under layer, and here consequently reate
directly on the white yolk.

To recapitulate : — In the normal unincubated hen's
egg we recognize the blastoderm, consisting of a com-
plete upper layer of smaller nucleated granular cells
and a more or less incomplete under layer of larger
cells, filled with larger granules ; in these lower cells
nuclei are rarely visible. The thin flat disc so formed
rests, at the uppermost part of the entire yolk, on a
bed of white yolk, and a peripheral thickening of the
lower layer causes the appearance in the blastodermic
disc of an area opaca and an area pellucida. The great
mass of the entire yolk consists of the so-called
yellow yolk composed of granular spheres. The
white yolk is composed of smaller spheres of pecu-
liar structure, and exists, in small part, as a thin
coating around, and as thin concentric laminae in
the substance of the yellow yolk, but chiefly in the

I.J THE OVARIAN OVUM.

tmn of a flask-shaped moss in the interior of the yol
tie npper somewhat expanded top of the neck
which foiTQS the bed on which the blastoderm restft;*
The whole yolli is invested with the vitelline mem-
brane, this again with the white ; and the whole is
I sovaed with two shell-membranes and a shell.

Such an egg has however undergone most important

ngea while still within the body of the hen; and

1 order to understand the nature of the structures

hrlicii have just been described, it will be necessary to

M briefly the history of the egg from the stage when

tt eiists as a so-called ovarian ovum in the ovary of a

.p to the time when it is laid.

In birds the left ovary alone is found in the adult ;

d is attached by the meaovarivm to the dorsal wall

pthe abdominal cavity, on the left side of the vertebral

It consists of a mass of vascular stroma in

h the ova are imbedded, is covered superficially

pftlayer of epithelium, continuous with the epithelial

ig of the peritoneal canity. The appearance of the

J varies greatly according to the age of the indi-

WuaL Id the mature and sexually active females

ptgalmoat wholly formed of pedunculated and highly

Bcular capsules of various sizes, each containing a more

pl«S3 developed ovum; in the young animal however

1 much more compact, owing to the absence of

pvaaced ova.

1 If one of the largest capsules of the ovary of a hen
Och in laying regularly be opened, it will be found to
a a nearly spherical (or more correctly, eUipsoidal
b but slightly unequal axes) yellow body enclosed in
plicate membrane. This is the ovaria.n ovutci ot

12 THE hen's egg. [char

Examined with care the ovum, which is tolerably uoi-*
form in appearance, will be found to be marked at one
spot (generally lacing the stalk of the capsule aud form-
ing the pole of the shorter axis of the ovum) by a small
disc differing in appearance from the rest of the ovum.
This disc which is known as the germinal disc or disout

Fio, 4.

a. ConDcctiTo-t issue capsule of tbe ovum, b, follicular epithe*
lium, at the surface of Trliick nearest tbe ovum lies tlia
ritaUiDe membrane, e. gmnulBr material of the germ
disc, which hecomeu cgnveftcJ iuto the bluatodem). (ThJi
is not very well represented in the woodcut. In section
which have been hardened in chromic acid it consists of fix
granules.) w. y, white yolk, which paaaea insonaibly int^
the fine granular material of the diso. x, germinal veaiolsi
enclosed in a distinct membrane, but shrivelled uji by ths
action of the chromic acid, t/, space originally complete
filled up by the germinal vesicle, before the latter ^
shrivelled up by the action of the chromic acid,

proligerus, consists of a lenticiilar mass of protoplasm
(Fig. 4, c), imbedded in which is a globular or ellipsoidal
body (Fig. 4, x), about 310/* in diameter, called the
germiTial vesicle. This has a delicate wall, and its c
tents are clear and fluid in the fresh state, but becom
granular upon the addition of reagents.

l] TBE OVARIAN OVUM. 13

The rest of the ovum is known as the yolk. This
conaista of two elements, the white yolk- and the yellow
jolk-spheres, which are distributed respectively very
rnuth m the same way as in the laid egg, the yellow
yolk fonning the main mass of the ovum, and the white
yolk being gathered underneath and around the disc
(^ig. *, w. y), and aJso forming a flask-shaped mass in
the interior. The delicate membrane surrounding the
whole is the vitelline membrane.

The youngest ova in the ovary of a fowl, in common
irith those of all other animals, present the characters
of sample cell. Such a cell is diagrammatically repre-
«iited in Fig. 5.

It is seen to consist of a naked protoplasmic body
containing in its interior a nucleus — the germinal vesi-
cle— which in its turn envelopes
?»■ S. a nucleolus — constituting what is

known as the germinal spot.
Such joi'ng ova are enclosed in
a capsule of epithelium, named
the follicle or follicular mem-
brane, and are irregularly scat-
tered in the stroma of the ovary.
The difference between such
an immature ovum -and the ripe
ovum just described is very great,
but throughout its growth the
ovum retains the characters of a
cell, so that the mature ova-
rian ovum, equally with the
Bgest ovum in the ovary, is a single cell.
:The most striking changes which takes place m ^a

THE HENS

couree of tlie maturation of the ovum concern the Wj
of the cell rather than the germinai veaicle. As &^
body grows in size a number of granules make theJ^
appearance in its interior. These granules are formed
by the inherent activity of the protoplasm, which i^ ,
itself nourished, in a large measure at any rate, by th^ i
cells of the follicle. The outermost layer of the protc>-^
plasm remains free from these granules. As the ovnn^l
grows older the granules become larger, first of all ii»- 1
the centre, and subsequently at the periphery, and tak9 1
the form of white yolk- spherules. The greater part ot '
them become at a later stage converted into yellow
yolk-spheres, while a portion of them, situated in the
position of the white yolk of the ripe ovum, retain their
original characters.

The germinal vesicle, which in the youngest ova is
eitoated centrally or subcentrally, travels in the course
of the growth of the ovum towards the peripheiy, and
the protoplasm immediately surrounding it remans
relatively free from yolk granules, and so constitutes
the germinal disc. In the younger ova there is but a
single germinal spot in the germinal vesicle, but as the
ova enlarge several accessory germinal spots make their
appearance, while in the ripe ovum it seems doubtful
whether there is any longer a trace of a germinal
spot.

The cells of the follicular epithelium are at first
arranged in a single row, but at a later stage become
two or more rows deep : they undergo however a
nearly complete atrophy in the ripe ovum. Around
the follicular epitheUum there is present a membrana
propria, and in the later stages of the growth of the

I.] THE OVAEUN OVUM.

ovum this is in its turn embraced by a highly vascular
connwHive-tissue capsule.

The youngest ova arc, as Las already been stated,
quite naked. In ova of about 1'5 mm. the superficial
]»J« of the ovum becomes coDverted into a radiately
«triated membrane called the zma radiata. At a later
period a second membrane, placed between the zona
fsdiata and the cells of the follicle, makes its appearance,
but its mode of origin is still unknown. As the ovum
ipproaches maturity the zona radiata disappears, and in
the ripe ovum the second membrane, which has already
Been spoken of as the vitelline membrane, alone
Bnaing.

I^m what has just been stated it follows that iu
•B e^ which has been laid the yolk alone constitutes
»* Inie ovum. The white and the shell are in fact
•ttessory structures formed during the passage of the
0 down the oviduct,

fhen the ovarian ovum is ripe and about to be
■cWged from the ovary, its capsule is clasped by
I open infundibulum of the oviduct. The capside
m bursts, and the ovum escapes into the oviduct, its
fciger axis corresponding with the long axis of the
iict, the germinal disc therefore being to oae

In describing the changes which take place in the
Muct, it will be convenient, following the order pre-
roualy adopted, to treat first of all of the formation
F the accessory parts of the egg. These are secreted
f the glandular walls of the oviduct. This organ
refbrc requires some description. It may bo said
It of four parts: — 1st, The dilated inl'undibulum

ted I

1 to

um A

THE hen's egg. [CH-VP.

with an ahdomiaal opemng. 2iid. A long tubular
portion — the oviduct proper — opening by a narrow neck
or isthmus into the 3rd portion, which is much dilated,
BDd haa been called the uterus ; the 4th part is some-
what narrow, and leads from the uterus into the cloaca
The whole of the mucous membrane lining the oviduct
is largely ciliated.

The accessory parts of the egg are entirely formed
in the Snd and 3rd portions. The layer of alhumeu
which immediately surrounds the yolk is first de-
posited ; the chalazffl are next formed. Their spiral
character and the less distinctly marked spiral firrange-
ment of the whole albumen is brought about by the
motion of the egg along the spiral ridges into which
the interior of the second or tubular portion of the
oviduct is thrown. The spirals of the two chalazse are
in different directions. This is probably produced by
their peripheral ends remaining fixed while the yolk to
which their centra! ends are attached is caused to
rotate by the contractions of the oviduct. During the
formation of the chalaziB the rest of the albumen is
also deposited ; and finally the shell -membrane is formed
in the narrow neck of the 2nd portion, by the fibrilla-
tion of the most external layer of albumen. The egg
passes through the 2nd portion in little more than
3 hours. In the 3rd portion the shell is formed. The
raucous membrane of this part is raised into nume-
rous flattened folds, like large villi, containing follicu-
lar glands. From these a thick white fluid is poured
out, which soon forma a kind of covering to the egg, in
which the inorganic particles are deposited. In this
portion of the oviduct the egg remiuna from 12 to 18

l] impregnation. 17

hours, during which time the shell acquires its normal
consistency. At the time of laying it is expelled from
the uterus by violent muscular contractions, and passes
ynih its nJow end downwards along the ^maindTof
the oviduct, to reach the exterior.

Impregnation, This process occurs in the upper
portion of the oviduct; the spermatozoa being found
actively moving in a fluid which is there contained.

We have as yet, as far as the fowl is concerned, no
direct observations concerning the changes preceding
and following upon impregnation ; nor indeed concern-
ing the actual nature of the act of impregnation.

In other types however these processes have been
followed with considerable care, and the result has been
to shew that prior to impregnation a division of the
ovum takes place into two very unequal parts. The
smaller of these parts is known as the polar body, and
plays no further part in the development In the
course of the division of the ovum into these two parts
the germinal vesicle also divides, and one part of it
enters the polar body, while a portion remains in the
larger segment which continues to be called the ovum,
and is there known as the female pronucleus. Im-
pregnation has been found to consist essentially in
the entrance of a single spermatozoon into the ovum,
followed by the fusion of the two. The spermatozoon
itself is to be regarded as a cell, the head of which
corresponds to the nucleus. When the spermatozoon
enters the ovum the substance forming its tail becomes
mingled with the protoplasm of the latter, but the head
enlarges and constitutes a distinct body called the Tnale
pronucleus, which travels towards and finally fuses with
P. A B. 2

18 THE HEN'B ego. [chap.

the female pronucleus to constitute the nucleus of the
impregnated ovum,

Segmontation. There follows upon the impregna-
tion a remarkable process known as the segmentation.
The process consists essentially in the division of the
impregnated ovum by a series of successive segmenta-
tions into a number of cells, of which the whole of the
cells of the future animal are the direct descendants.
In the majority of instances this process results in the
division of the whole ovum into cells ; but in cases of
ova where there is a large amount of food yolk, only
that part of the ovum in which the protoplasm is but
ahghtly loaded with food material, and which we have
already described as the germinal disc, becomes so
divided. The remainder of the ovum constitutes a
food reservoir for the use of the developing embryo
and is known as the food yolk. The segmentation in
such ova, of which that of the Ibwl ia one of the
best known examples, is described as being partial or
meroblastic'.

In order to understand the process of segmentation
in the fowl's ovum it must be borne in mind that the
germinal disc is not sharply separated from the re-
mainder of the ovum, but that the two graduate insen-
sibly into each other.

The segmentation commences in the lower part of
the oviduct, shortly before the shell has begun to be
formed.

Viewed from above, a furrow is seen to make its

' For a Fnller ncimiiit of the relaUan between boloblastio md
merobtnBtio BegmeatatLon the reader is referred lo the treatise on
Compaiative Embryology Lj Balfour, Vol. i. ohapter iii.

I Sdujus Tizws or the zarlt SrAass of thb Seouentation
Di A Fotl'b Esq. (A and C after Coata.)

i npreeents the earliest stage. The first fiirrow (6) has
m to moke its eLppeoronce in the cectie of the genoiiial diao,
ttpaipheiy ia mftrked by the line a. In S, the first furrow
i aearlj across the diao, and a second similar furrow
0 the first has appeared. The disc thus
ided somewhat irregularly into qiiadranta by four
In a. later etage (C) the meridian furrows b have
1 number, from four, as ia £, to nine, and cross
rt have nlao made their appearance. The disc is thus cut
0 anutll central (c) and larger peripheral (d) segmenta.
IS furrows are aeen juat beginuiug, as ex.ffr, close
ie end of the line of reference d.

mce, running across the germinal disc, though
t for the whole breadth, and dividing it into two
. 6, A). This primary furrow U succeeded
a second at right angles to itself. The surface thus

B divided into four segments or quadrants (Fig.

a— 1

Tlie second furrow cute the first somewhat excea-
tricallj.

The first four furrows do not extend through the
whole thickness of the genninal disc, and the four seg-
ments marked out by them are not separated from the
disc on their lower aspect,

Elach of these is again bisected by radiating furrows,
and thus the number of segments is increased from four
to eight (it may be seven or nine). The central portion
of each segment is then, by a cross furrow, cut off from
the peripheral portion, giving rise to the appearance of
a number of central smaller segments, surrounded by
more external elongated segments (Fig. 6, C).

The excentricity in the arrangement of the segments
is moreover still preserved, the smaller segments being
situated nearer one side of the germinal disc. The
excentricity of the segmentation gives to the segmenting
genninal disc a, bilateral symmetry, but the relation
between the axis of symmetry of the segmenting germinal
disc and the long axis of the embryo is not know-u.

Division of the segments now proceeds rapidly by
means of furrows running in various directions. And
it is important to note that the central segments
divide more rapidly than the peripheral, and con-
sequently become at once smaller and more numerous
(Fig- 7).

Meanwhile sections of the hardened blastoderm
tiiiach us that segmentation is not confined to the sur-
face, but extends through the mass of the blastoderm ;
they shew us moreover that division takes placo by
means of not only vertical, but also horizontal furrows,
i.e. furrows parallel to the suri'ace of the disc (Fig. 8).

SEGMENT A.TIOH.
Pw. 7.

^''•'Wg ViETT OF TBX Qekhinal Di3c OF A Hen's Eoa
DOEmo THE LATER Stages of SEoiiENTAtiOM.
{Chromic AeiJ Preparation.)
At e in the centre of the diac the segmeDtatiou masses are
^ «mail and numerous. At 6, nearer the edge, thej are
*'K""md fewer; while tliose at the extreme margin a are largest
fewest of all. It will be noticed that the radiating furrows
^'fa'ig off the segments a do not reach to the extreme margin
•"f ttw disc.

"IB drawing is completed in one quadrant only ( it wiO of
be understood tbivt the whole circle ought to he filled up
'•piBdaely similar manner.

In this way, by repeated division or segmentation,
* original germinal disc is cut up into a large number
Knall rounded masses of protoplasm, which are small-
t u the centre, and increase in size towards the peri-
Wy. The segments lying uppermost are moreover
Uler than those beneath, and thus the establishment
two layers of the blastoderm is foiealiado'weA.

Sbotios of tke Germinal Dieo of a Fowl dcbinq thb
Later Staoes of Segmentation.
The Hection, which represents rather more than half the
breadth of the blcwtodenii (the middlo line being shewn at c),
aliewa that the upper and central parts of the disc Begtnent
bster than those below and towards the periphery'. At the
periphery the segments are still very large. One of the larger
segmentB is shewn at a. In the majority of segments a cuoleus
can be seen ; and it seems probablo that a nucleus is present in
alL Most of the segments are filled with highly refracting
Bpherules, but these are more numerous in some cells (eapeoially
the larger cells near the yolk) than in others. In the cantiD]
part of the blastoderm the upper cells have commenced to form
a distinct layer.

a. large peripheral celL b. larger cells of the lower parts of the
blastoderm, e. middle line of blastoderm, e. edge of the
blaatodarm adjoining the white jolk if. white yolk.

In the later stages of segmentation not only do the
first-formed segments become further divided, but seg
mentation also extends into the remainder of the germi-
nal disc.

The behaviour of the nucleus during the segmenta-
tion haa not been satisfactorily followed, but there ie.

SEGMENTATION. 23

from the analog; of other forms, no doubt that in the
Einnatioa of the first two Begments the original nucleus,
formed by the fusion of the male and female pronuclei,
beoomes divided, and that a fresh division of the nucleus
tikes place with the formation of each ii-esh segment.
Nuclei make their appearance moreover in the part of
the ovum immediately below that in which the segmen-
tation has already taken place ; these are in all proba-
"ih^ also derived from the primitive nucleus. The
aibstance round some of these nuclei rises up in the
fctm of papillffi, which are subsequently constricted off
■id set free as supplementary segmentation masses;
■irtiile some of the nuclei remain and form the nuclei
■uEady spoken of as existing in the bed of white yolk
kIow the blastoderm in the unincubated egg.

etween the segmented germinal disc, which we
now call the blastoderm, and the bed of white yolk
Bi which it rests, a space containing fluid makes its

Aa development proceeds, segmentation reaches ita
lite in the centre, but continues at the periphery, and
18 eventually the masses at the periphery become of
!Mme size as those in the centre.
The distinction however between an upper and a

'Ser layer becomes more and more obvious.
The masses of the upper layer arrange themselves,
3 by side, with their long axes vertical ; their nuclei
very distinct. In fact they form a membrane

'columnar nucleated cella.
The masses of the lower layer, remaining larger than
B of the upper layer, continue markedly granular
round, and form rather a close irregular nelvotV

24 THE hen's egg. [chap. I.

than a distinct membrane. Their nuclei are not readily
visible.

At the time when the segmentation-spheres in the
centre are smaller than those at the periphery, and
those above are also smaller than those below, a few
large spherical masses, probably containing each one of
the nuclei already spoken of, arise by a process of seg-
mentation from the bed of white yolk, and rest directly
on the white yolk at the bottom of the shallow cavity
below the mass of segmentation-spheres. They contain
either numerous small spherules, or fine granules;
the spherules precisely resembling the smaller spheres
of white yolk. These loose spherical masses form the
majority of the formative cells already spoken of.

Thus the original germinal disc of the ovarian ovum
becomes, by the process of segmentation, converted into
the blastoderm of the laid egg with its upper layer of
columnar nucleated cells, and its lower layer of irregu-
larly disposed cells, accompanied by a few stray " forma-
tive " cells lying loose in the cavity below.

CHAPTER IL

A BEIEF SmMAKT OF THE WHOLE HISTOBT OF
INCUBATION.

Step by step the simple two-layered blastoderm
dttoibed in the previous chapter is converted into the

iplex organism of the chick. The details of the
BiMiy changes through which this ead is reached will
ftthaps be rendered more inteUigible if we prefix to the
ifaial history of them a brief summary of the general
Wuise of events from the beginning to the end of incu-

In the first place. It is to be borne in mind that the
■luljiyo itself is formed in the area pellucida, and in the
pellucida alone. The area opaca in no part enters
■irectly into the body of the chick; the structures to
'lich it gives rise are to be regarded as appendages,
fhich sooner or later disappear.

0«miinal layers. The blastoderm at starting con-
•ts of two layers. Very soon a third layer makes its
^arance between the other two. These three layers,
Mwn as the germinal layers, the establishment of which
'« (act of fimdamental importance in the history of the
ibryo, are called respectively tho upper, middle and
\af&s, or epiMast, mesohlaat and feypublost. O^

26 PEELmiNAJiY ACCOUNT. [CHAP.

these the epiblast and hypoblast conatitute the primary
layers.

Three similar germinal layers are found in the
embryos of all vertebrate and most invertebrate forma,
and their history is one of the most important parts of
comparative embrj-ology.

The epiblast gives rise to the epidermis, the central
and peripheral parts of the nervous system, and to the
most important parts of the organs of special sense.
The hypoblast is essentially the secretory layer, and
furnishes the whole epithelial lining of the alimentary
tract and its glands, with the exception of part of the
mouth and anus which are lined by the epiblast and
are spoken of by embryologists as the sfomodtmtm and
proctodwum. Finally the mesoblaat is a source from
which the whole of the vascular system, the muscular
and skeletal system, and the connective tissue of all
parts of the body, are developed. It gives in fact origin
to the connective -tissue basis both of the skin and of
the mucous membrane of the alimentary tract, and to
all the structures lying between these two with the
exceptions already indicated. It is more especially to
be noted that it gives rise to the excretory organs and
generative glands.

Formation of the embryo. The blastoderm which
at first, as we have seen, lies like a watch-glass over the
cavity below, its margin resting on the circular germinal
wall of white yolk, spreads, as a thin circular sheet, over
the yolk, immediately under the vitelline membrane.
Increasing uniformly at ail points of its circumference,
the blastodermic expansion covers more and more of the
yolk, and at last, reaching the opposite pole, completely
envelopes it. Thus the whole yolk, instead of being

THE HEAD-FOLD.

losed as formerly by the vitelline membrane alone,
Des to be also enclosed in a bag formed by the blasto-

It is not however until quite a late period that the

lete closing in at the opposite pole takes place ; in

rt the extension of the blastoderm must be thought

fa going on during the first seven days of incubation.

Both the area opaca and the area pellucida share lu

lis CDlargement, but the area opaca increases much

ore rapidly than the area pellucida, and playa the

lincipal part in encompassing the yolk.

The meaoblast, in that part of the area opaca which

nearest to the area pellucida, becomes the seat of

Bcaliar changes, which result in the formation of blood-

iBiela. Hence this part of the area opaca ia called the

veular area.

The embryo itself may be said to be formed by a
Uing o£f the centra! portion of the area pellucida from
IB rest of the blastoderm. At first the area pellucida
(juite flat, or, inasmuch as it forms part of the circum-
Pence of the yolk, slightly but uniformly curved. Very
Od, however, there appears at a certain spot a semi-
Mr groove, at first small, but gradually increasing in
and extent; this groove, which is represented in
Kion in the diagram (Fig. 9, A), breaks the uni-
lity of the level of the area pellucida. It may be
•en of as a tucking in of a small portion of the
todenn in the form of a crescent. When viewed
I above, it presents itself as a curved line (the hinder
10 two concentric curved lines in front of A in Fig.
which marks the hind margin of the groove, the
presEion itself being bidden.

PRELIMINARY ACCOUNT.

[chap.

Fio. 9.

A /

^K
^\--''

ve.

-*p

Fig. 9,AioIi forms a series of purelj diagrammatio repre-
sentations introduced to facilitate the comprehension of the
manner in which the body of the embryo is formed, and of the
various relations of the yolk-sac, amnion and allantois.

In all v^ is the vitelline membrane, placed, for convenienoe
sake, at some distance from its contents, and represented as per-
sisting in the later stages ; in the actual egg it is in direct contact
with the blastoderm (or yolk), and early ceases to have a separate
existence. In all e indicates the embryo, pp the general pleoro-
peritoneal space, of the folds of the amnion proper ; ae or ae the
cavity holding the liquor amnii ; al the allantois ; a the ali-
mentary canal ; y ovys the yolk or yolk-sac.

A^ which may be considered as a vertical section taken longi-
tudinal!^ along the axis of the embryo, represents the relations of
the {Mirts of the egg at the time of the first appearance of the
head-fold, seen on the right-hand side of the blastoderm e. The^

THE EMBRTOlflC APPENDAGES, ""2^™

itcdenn is spreading both behind (to the left hand in the

re), &□<! in front (to right hand) of the head-fold, ita limits

igiodicated by the shading and thickening for a. certain dis-

X of the miu^tn of the yolk t/. _ As jet there is no fold on the

side of e eorreaponding to tho hcad-fold on the right.

Bias, TBrtical tranaveise eeoUon. of the aame period drawn

aoQTeiiienae aake on a larger auale (it should have been made

■tier tnd less curved). It shews that the hlostoderm (vertically

Atded) is eitending laterally as well as fore and oft, in fact in

dincttons ; but there are no lateral folds, and therefore do

ati limita to the body of the embryo as distinguished fron)

btblutadenn.

Inddentally it shews the formation of the medullary grcove

rttw rising up of the lamjnn dorsoles. Beneath the section of

■ groove is seen the rudiment of the notochord. On either side

>lbg iodicates the cleavage of the mesoblast just commencing.

In C, nhich represents a vertical longitudinal aet^tion of later

«, both head-fold (on the right) and tjul-fold (on the left) have

IncKiMl considerably. The alimentary canal ia therefore closed

nth in &oot and behind, but is in the middle atiU widely

to the yolk y below. Though tho axial parts of the embryo

become thickened by growth, the body-walla are still thin ;

en however is seen the cleavage of the mesoblost, and the

g»Me of the Honiatopleure and splanchnopleure. The

<^lsopleiure both at the head and at the tail is folded in to

PtMer aitent than the somatopleure, and forms the still wide

stalk. At the end of the stalk, which is as yet short,

wis outwarda again and spreads over the surfato of the yolk.

■> umatopleure, folded in less than the splanchnopleure to

tbe wider somatic atalk, sooner bends round and runs out-

■■lla^iD. At a little distance from both the head and the

Bitii raised up into a fold, a/, a/, that in front of the head

igthe highest. Theseore the amniotic folds. Descending from

vfold, it speedily joins the splanchnopleure again, and the

I once more united into an uncleft membrano, eitend some

dtmiwarda over tho yolk, the limit or outer margin of the

being shewn. All the space between the soma-

splanchnopIcuTi^ pp, is shaded with dots. GLoau

PBEIJMINAET ACCOUST.

[chap.

to the bodj tiiis space may be called the pleuroperitoneal cavitf ;
but outside the bodjr it nma up iato either aumiotio fold, end
also exteDds some Uttle vaj over the yolk.

J> repreaenta the tail end at about the Bame stage on a more
enlarged scale, io order to illustrate the position of the aUantois
al (which was for the sake of simplicity omitted in C), shewn as a
bud teoia the splanctmopleure, stretching downwards into the pleu-
mperitoneal cavity pp. The dotted area representing as before thfl

TEB KKBBTOICIC APPSNDA.GES.

leiptce between the spUnchDopIeure and the somatopleon,
etjAeat Uut a wajr ia open for the allantois Ut extend from
pnsent poeitioti into the space between the two hmbs of the

fold a/

£, alto ft kragittidinal section, represents a stage stiU foitber
laeed. Both splanchnic and somatic ataiks are much nar-
■d, tfpedallj the former, tiie cavity of the aliment*!^ canal
Know connected with the cavity of the jo!k-aack by a mere
■L The folds of the amnion are Hpreadiog over the top of
tmbtyo and nearly meet. Each fold consisis of two walla
bibe, the ^lace between which (dotted) b as before merely
ttt of the apace between the somatopleure and splandino-
BB. Between these arched amniotic folds and the body of
<aml)tyi> is a space not as yet entirely closed in.
/ npneenta on a difierent scale a transveree section al E
tfaroogh the middle of the splanchnic stalk. The daric ring
Ik body of the embryo shewd the positioa of the neural canal,

■ «tiich is a black spot, marking the notochord. On either
I of the notochord the divei^noe of somatopleure and splonch-
iNrais obvious. The sploachnopleure, more or leaa thick-
I, '» somewhat bent in towards the middle line, but the two
I do not unite, the alimentary canal heuig as yet open below

bit qtot ; after converging somewhat Uiey diverge again and
ootwards over the yolk. The somatopleure, folded in to
leitttit to form the body-walla, soon bends outwards again,
is almost immocliately raised up into the lateral folds of the
im af. The continuity of the pleuroperitoneal cavity within
i^y with the interior of the amniotic fold outside the body

; both cavities are dotted.
8, which corresponds to A at a later stage, ia introduced to
( the manner in which the allantois, now a distinctly hollow
r, «boee cavity is continuous with that of the alimentary
1^ becomes dii^cted towards the amniotic fold.
[d J/ a longitudinal, and / a transverse section of later date,
t changes have taken place. The several folds of the amnion
) met and coalesced a)>ove the body of the embryo. The inner

■ of ^e several folds have united into a. single membrane {a),
ac) round the embryo. This mem-

PRELIMINARY ACCODIJT.

brane (a) is the amnion proper, and the cavity within it,

it aad the embryo, is the canity of the amnion cootainiiigllia

liqaor amniL The allantois is omitted for the sake of tdn-

plicitj.

It will be seen that the amnion a now forms in every direc-
tion the termination of the somatopleure ; the peripheral portioga
of the somatopleure, the united outer or descending limbs ofttw
folds a/ in G, J), F, G having been cut adrift, and now forming
an independent oontinuoua membrane, the strout mea^rOM,
immediately underneath the vitelline membrane.

In / the aplanchnopleure ia seen converging to complete U»
closure of the alimentary canal a' even at the stalk (elaewhm
the canal has of course loug been closed in], and thea spreading
outwards as before over the yolk. The point at which it uoitn
with the somatopleure, marking the extreme limit of thecleavsga
of the mesoblast, ia now much nearer the lovrer pole of tin
diminished yolk.

IL] the EICBRTONIC APPENDAGES. 33

Ab a result of these seyeral changes, a great increase in the
dotted space has taken place. It is now possible to pass from
the actual peritoneal cavity within the body, on the one hand
round a great portion of the circumference of the yolk, and on the
other hand above the anmion a, in the space between it and the
serous envelope.

Into this space the allantois is seen spreading in K &ial.

In L the splanchnopleure has completely invested the yolk-
aac^ but at the lower pole of the yolk is still continuous with
that peripheral remnant of the somatopleure now called the
serous membrane. In other words, the cleavage of the mesoblast
has been carried all round the yolk (y«) except just at the lower
pole.

In M the cleavage has been carried through the pole itself ;
the peripheral portion of the splanchnopleure forms a complete
investment of the yolk, quite unconnected with the peripheral
portion of the somatopleure, which now exists as a continuous
membrane lining the interior of the shelL The yolk-sac (y«) is
therefore quite loose in the pleuroperitoneal cavity, being con-
nected only with the alimentary canal (a') by a solid pedicle.

Lastly, in Ji the yolk-sac. (y«) is shewn being withdrawn into
the cavity of the body of the embryo. The allantois is as before,
for the sake of simplicity, omitted ; its pedicle would of course lie
by the side of y« in the somatic stalk marked by the usual dotted
shading.

It may be repeated that the above are diagrams, the various
spaces being shewn distended, whereas in many of them in the
actual egg the walls have collapsed, and are in near juxta-
position.

In a vertical longitudinal section carried through the
middle line, we may recognize the following parts (Fig.
9, A, or on a larger scale Fig. 10, which also shews details
which need not be considered now). Beginning at what
will become the posterior extremity of the embryo (the
left-hand side of the figure in each case), and following the
suibuce of the blastoderm forwards (to the right in the

IP.AB, 3

PBEUMIMARY ACCOUNT, [CHAT.

DuattAUiiATio LoNQiTDDisAL Sectios THKonaa the Aim *
AN Bmbbyo.

The section ia auppoaed to be made at a time wbcn the btii-

Ibid hoa commenced but tbe tali-fold has not jet appeared.

J^. So. fold of the aomatopleure.

F. ,S>, fold of tho aplancliDopleaTa.

The line of reference F. So. is placed in the lower bay, ofitsid*

the embT7o. The line of I) is placed in the upper haj inralt

the embryo ; this nill remain as the ohmentary caaei. Botb

folds (/". So., F. Sp.) are parta of the head-fold, and are to be

thought of as continually travelling onwards (to the left)

velopment proceeds.

pp. space between somatoploore and aplanchnopleure : pUow-

peritoneal cavity.
dm. oommenoing (hem!) fold of tbe amm'on.
A fuller explanation la given under Fig. S9.

figureB), the level is maintained for some distance, aod
then there is a sudden descent, the blasto'derm bending
round and pursuing a precisely opposite direction to
previous one, running backwards instead of forwards, ft
some distance. It soon, however, turns round again,
once more running forward, with a gentle ascent, regain
the original leveL As seen in section, then, the blaato
derm at this spot may be said to be folded up in th

|L] the head-fold. 35

form of the letter 3. This fold we shall always speak of
M the head-fold. In it we may recognize two limbs:
in upper limb in which the curve is directed forwarda,
Q.i its bay, opening backwards, is underneath the blas-
--ierm, i.e. as we shall see, inmie the embryo (Fig, 10.
b/; and an onder limb in which the curvo is directed
backwards, and its bay, opening forwards, is above the
i)laatoderm,i.e. outside the embryo. If an 8 like the above,
luade of some elastic material, were stretched laterally,
the effect would he to make both limbs longer and
proportionally narrower, and their bays, instead of being
ihiJlow cups, would become more tubular. Such a
Wiult is in part arrived at by the growth of the blasto-
ierai; the upper limb of the g is continually growing
forward (but, imlike the stretched elastic model,
CRMea in all its dimensions at the same time), and the
lower limb is as continually lengthening backwards;
Mid thus both upper and lower bays become longer and
longer. This we shall hereafter speak of as the travel-
ling backwards of the head-fold.

The two bays do not however both become tubular.
Tile section we have been speaking of is supposed to be
'*ien vertically along a line, which will afterwards be-
-OKB the axis of the embryo; and the lower bay of the
'i 13 a section of the crescentic groove naentioned above,
in its middle or deepest part. On cither side of the
niidiile line the groove gradually becomes shallower.
Beatx in sections taken on either aide of the middle
fine or azis of the embryo (above or below the plane
of the figures), the groove would appear the less marked
the farther the section from the middle line, and at a
estoin distance would disappear altogether. Ilnn]>a\.\i&
Bl -6—%

■

PKELIMINAR'jf ACCOUNT.

[chap.

at first creacent-shaped,

remembered that tbe groove i

with the concavity of the crescent turned towards what
will be the hind end of the embryo (Fig. 22). As the whole
head-fold is carried farther and farther back, the honu)
of the crescent are more and more drawn in towards the
middle line, the groove becoming first semicircular, then
horse-shoe-shaped. In other words, the head-fold,
instead of being a simple fold running straight back-
wards, becomes a curved fold with a central portion in
front running backwards, and two side portions running
in towards the middle line. The effect of this is that
the upper bay of the 8 (that within the embryo) geta
closed in at the sides as well as in the front, and thua
speedily becomes tubular. The under bay of the 8
(that outside the embryo) remains of course open at the
sides as in front, and forms a sort of horse -shoe-shaped
ditch surrounding the front end of the embryo.

We have dwelt thus at length on the formation of
the head-fold, because, unless its characters are fairly
grasped, much difficulty may be found in understanding'
many events in the history of the chick. The reader
will perhaps find the matter easier to comprehend if he ■
makes for himself a rough model, which he easily can
do by spreading a cloth out flat to represent the blasto-
derm, placing one hand underneath it, to mark the s
of the embryo, and then tucking in the cloth from above
under the tips of bis fingers. The fingers, covered with
the cloth and slightly projecting from the level of the
rest of the cloth, will represent the head, in front of
whicli will be the semicircular or horae-shoe-ahapect
groove of the head-fold.

At its first appearance the whole S may be Bpc^ett

] THE TAIL-FOLD. 37 I

K the bead-fold, but later on it will be found con- 1

Dient to restrict the name cbiefiy to the lower limb I

Some tjine after the appearance of the head-fold, an
egether similar but at first less conspicuous fold
lies its appearance, at a point which will become the
iterior end of the embryo. This fold, which travels J
wards just as the bead-fold travels backwards, is the r
i/dd (Fig. 9, G).
In addition, between the head- and the tail-fold two '
sal folds appear, one on either side. These are '
ipler in character than either head-fold or tail-fold,
aunch as they are nearly straight folds directed I
Uranla towards the axis of the body (Fig. 8, F), and not J
located by being crescentic in form. Otherwise they j

eucUy similar, and in fact are fonned by the con-
Inations of the head- and tad-folds respectively.
As these several folds become more and more de-
bped, the head-fold travcUing backwards, the tail-
B forwards, and the lateral folds inwards, they tend to i
He in the middle point ; and thus give rise more and
le distinctly to the appearance of a small tubular ,
Rated upon, and connected, by a continually-nar- '
(ring hollow stalk, with that larger saa which is fonned .
Bie extension of the rest of the blastoderm over the J
ole yolk.

The smaller sac we may call the " embryonic sac," \

'larger one " the yolk-sac." As incubation proceeds,

! smaller sac (Fig. 9) gets larger and larger at the

WpensG of the yolk-sac (the contents of the latter being <

gfaJuaJIy assimilated by nutritive processes into the

tanies forming the growing walls of the former, not J

38 PRELIMISABT ACCO0NT. [CilAP.

directly transferred from one cavity into the gther).
Within a day or two of the hatching of the chick, at a
time when the yolk-sac is still of some considerable size,
or at least has not yet dwindled away altogether, and
the development of the embryonic sac is nearly com-
plete, the yolk-sac (Fig. 9, N) is slipped into the body
of the embryo, bo that ultimately the embryonic sac
alone remains.

The embryo, then, is formed by a folding-off of a
portion of the blastoderm from the yolk-sac. The
general outline of the embryo is due to the direction
and shape of the several folds which share in its forma-
tion ; these, while preserving a nearly perfect bilateral
symmetry, present marked differences at the two ends
of the embryo. Hence from the very first there is no
difficulty in distinguishing the end which will be- the
head from that which will be the tail.

In addition to this, the tubular sac of the embryo,
while everywhere gradually acquiring thicker and
thicker walls, undergoes at various points, through local
activities of growth in the form of thickenings, ridges,
buds or other processes, many modificationB of the
outhne conferred upon it by the constituent folds. Thus
bud-like processes start out from the trunk to form the
rudiments of the limbs, and similar thickenings and
ridges give rise to the jaws and other parts of the face.
By the unequal development of these outgrowths the
body of the chick is gradually moulded into its proper
outward shape.

Were the changes which take place of this class
only, the result would be a tubular sac of somewhat com-
phcated outline, but stUl a simple tubular sac. Such

THE MEDULLARY CANAL.

ample sac might perhaps be roughly taken to rcpre-
it the body of many an iuvertebrate animal ; but tha
acal alructure of a bird or other vertebrate anima! is
lely different. It may very briefly be described a«'

Hows.
First there is, above, a canal running lengthways
1^ the body, in which are lodged the brain and
lal oord. Below this neural tube is an asis repre-
ted by the bodies of the vertebrse and their con-
ution forwards in the structures which form the base

itteskulL Underneath this, again, is another tube

Iwd in above by the axis, and on the sides and below
the body-walls. Enclosed in this second tube, and

qiendod from the axis, is a third tube, consisting of
liimentary canal with its appendages (liver, pan-
iB, langs, &c., which are fundamentally mere diver-
lia from one simple canal). The cavity of the outer
e, which also contains the heart and other parts of
vascular system, is the general body cavity ; it con-
>af a thoracic or pleural, and an abdominal or peri-
od section ; these two parts are, however, from their
le of origin, portions of one and the same tub*'.
U a transverse sectiou of a vertebrate animal always
W the same fundamental structure : above a single
B, below a double tube, the latter consisting of one
i enclosed within another, the inner being the ali-
itary canal, the outer the general cavity of the body.
iBuch a triple tube the simple tubular embryonic
of the chick is converted by a series of changes of a

rkable character.
The upper or neural tube is formed in the foUovring
AA b very early period the upper layer of tl

40 PRELIMINABT ACCOUNT. [CHAP.

blastoderm or epiblast in the region which will become
the embryo, is raised up into two ridges or folds, which
run parallel to each other at a short distance on either
aide of what will be the long axis of the embryo, and
thus leave between them a shallow longitudinal groove
(Fig. 9, B, also Figs. 21, m.c). As these ridges, which
bear the name of medullary folds, increase in height
they arch over towards each other, and eventually meet
and coalesce in the middle line, thus converting the
groove into a canal, which at the same time becomes
closed at either end (J'ig. 8, F. I, also Fig. 34. Mc).
The cavity so formed is the cavity of the neural tube,

I and eventually becomes the cerebro-spinal canal. Its
walls are wholly formed of epiblast.

The lower double tube, that of the alimentary canal,
and of the general cavity of the body, is fonued in an
entirely different way. It is, broadly speaking, the
result of the junction and coalescence of the funda-
mental embryonic folds, the head-fold, tail-fold, and
lateral folds ; in a certain sense the cavity of the body
is the cavity of the tubular sac described in the last
paragraph.

But it is obvious that a tubular sac formed by the
folding-in of a single sheet of tissue, such as we have
hitherto considered the blastoderm to be, must be a
simple tubular sac possessing a single cavity only. The
blastoderm however does not long remain a single
sheet, but speedily becomes a double sheet of such a
kind that, when folded in, it gives rise to a double
tube.

Very early the blastoderm becomes thickened in the
region of the embryo, the thickening being chie&y due

THE BODY CAVITY.

n increase in the middle layer or mesoblost, whili
It the Goine time it becomes split or cleft horizoatalln
over tile greater part of its extent into two leaves, t

3r leuf aud a tower leaf. In tlie neighbourhood of 1
fte axis of the body, beneath the neural tube, thia' 1
deavage is absent (Fig. 9, B ; also Figs. 2*, 34), in fact,
It begins at some little distance on either side of the
■liiaDd spreads thence into the periphery in all direc-
tions. It is along the mesoblast that the cleav^e
likes place, the upper part of the mesoblast uniting
liitb epiblast to form the upper leaf, and the lower
rt with the hypoblast to form the lower leaf
In the fundamental folds both leaves are involved,
1 leaves are folded downwards and inwards, both
*Te8 tend to meet in the middle below ; but the
>*er leaf is folded in more rapidly, and thus diverges
m the upper leaf, a space being gradually developed
^^ t»een them (Fig. 9). In course of time the several
Sdda of the lower leaf meet and unite to form an inner
ie quite independently of the upper leaf, whose own
Wm in turn meet and unite to form an outer tube
•panted from the inner one by an intervening space,
s inner tube which from its mode of formation is
«ly lined by hypoblast is the alimentary canal which
I nbaeqaently perforated at both ends to form the
BQth and anus ; the walls of the outer tube are the
iUb of the body ; and the space between the two tubes
i the general hodi/ or pleuroperitoneal cavity.
Hence the upper (or outer) leaf of the blastoderm,
1 its giving rise to the body-walls, is called the
natopleure ' ; the lower (or inner) leaf, from its form-
' 8oma,bodj,pU»nm,eiie.

4& prei.:minaky accodnt. [chap.

ing the alimentary cajial and its tributaiy viscera;, the
splanchnopleure '.

This horizontal splitting of the blastoderm into a
somatopleure and a splanchnopleure, which we shall
hereafter speak of as the cleavage of the mesoblatt, is
not confined to the region of the embryo, but gradually
extends over the whole of the yolk-sac. Hence in the
later days of incubation the yolk-sac comes to have
two distinct coats, an inner aplanchnopleuric and an
outer somatopleuric, separable from each other all
over the sac We have seen that, owing to the
manner of its formation, the ' embryonic sac ' is con-
nected with the ' yolk-sac ' by a continually narrowing
hollow stalk ; but this stalk must, like the embryonic
eac itself, bo a double stalk, and consist of a smaller
inner stalk within a larger outer one, Fig, 9, E, H,
The folds of the splanchnopleure, as they tend to
meet and unite ia the middle line below, give
rise to a continually narrowing hollow stalk of their
own, a splanchnic stalk, by means of which the wfJls of
the alimentary canal are continuous with the splanch-
nopleuric investment of the yolk-sac, and the interior
of that canal is continuous with the cavity inside the
yolk-sac. In the same way the folds of the somato-
pleure form a similar stalk of their own, a somatic
stalk, by means of which the body-walls of the chick
are continuous (for some time ; the continuity, as we
shall see, being eventually broken by the development
of the amnion) with the somatopleoric investment of
the yolk-sac ; and the pleuroperitoneal cavity of the

* SpUatehnon, i

B, pUvron, Bidp.

Wyof the chick is continuous with the narrow apace
betffWD the two investments of the yolk-sac.

At a comparatively early period the canal of the
iplanchnic stalk becomes obliterated, so that the
louterial of the yolk can no longer pass directly into
the alimentary cavity, but haa to find its way into
tiie body of the chick by absorption through the blood-
Tesaels. Tlie somatic stalk, on the other hand, remains
lidelj open for a much longer time ; but the somatic
"lell of the yolk-sac never undergoes that thickening
"hieli takes place in the somatic walls of the embryo
Itielf; on the contrary, it remains thin and insignificant,
"hen, accordingly, in the last days of incubation the
pwtiy diminished yolk-sac with its splanchnic invest-
"wit is withdrawn into the rapidly enlarging abdominal
**rity of the embryo, the walls of the abdomen close
a ud unite, without any regard to the shrivelled,
*lptied somatopleuric investment of the yolk-aac,
™di is cast off as no longer of any use. (Fig. 9. Com-

' the series.)

The Anmion. Very closely connected with the
■esTage of the mesoblast and the division into soma-
^'pleure and splanchnopleure, is the formation of the
1, all mention of which was, for the sake of
■oplicity. purposely omitted in the description just

The amnion is a peculiar membrane enveloping the
ibryo, which takes its origin from certain folds of
i somatopleure, and of the somatopleure only, in the
allowing way.
At a time when the cleavage of the mesoblast has
lewhat advanced, there appears, a little way in front

M PRELIMINAEY ACCOUNT. [CHAP.

of the semilunar head-fold, a second fold (Fig. 22, also
Pig. 9, C), running more or less parallel or rather con-
centric witii the first, and not unlike it in gener&l
appearance, though diflering widely from it in nature.
In the head-ibid the whole thickness of the blastoderm
is involved ; in it both somatopleure and splanchno-
pleure (where they exist, i.e. where the meaoblast is
cleft) take part. This second fold, on the contrary, is
limited entirely to the somatopleure. Compare Figs.
9 and 10. In front of the head-fold, and therefore alto-
gether in front of the body of the embryo, the somato-
pleure is a very thin membrane, consiating only of
epiblast and a very thin layer of mesoblast ; and the
fold we are speaking of is, in consequence, itself thin
and delicate. Kising up as a semiluDar fold with its
concavity directed towards the embryo (Fig. 9, 0, of),
as it increases in height it is gradually drawn back-
wards over the developing head of the embryo. The
fold thus covering the head is in due time accompanied
by similar folds of the somatopleure starting at some
little distance behind the tail, and at some little dis-
tance from the sides (Fig. 9, C, D, E, F, and Fig. 11 am.).
In this way the embryo becomes surrounded by a
series of folds of thin somatopleure, which form a con-
tinuous wall all round it. All are drawn gradually
over the body of the embryo, and at last meet and
completely coalesce (Fig. 9, H, I), all traces of their
junction being removed. Beneath these united folds
there is therefore a cavity, within which the embryo
lies (Fig. 9, H, ae). This cavity is the cavity of the
amnion. The folds which we have been describing are
those which form the amnion.

THE AMNION.

Via. n.

T&BOUQH TBH FOft-

EuBETO Bird, ±1 thb timb or the
Kbkatios of the Allantoih.

9^ cp~blaBt ; Sp.c. spinal canal ; eh. notocbord ; n.e. neurenteric
nnal ; ^. hjpoblABt ; p.o-g. postanal gut ; pr, romaios of
primitive streak folded in on the ventral side ; al. allaatois ;
■U. mcfioblast; ari. point whore anus will be formed; p.c.
perivisceral cavity ; am. amnion ; to. aomatopleure ; «p.
•C^chnopleure.

Each fold, of course, necessarily conaista of two
™'b8,botb limbs consisting of epiblast and a very thin
■JBT of mesoblaat ; but in one limb the epiblast looks
*Warda the embrj-o, while in the other it looks away
"Wu it The space between the two limbs of the fold,
" Can easily be seen in Figs. 9 and 11, is really part
w the space between the somatopleure and splanch-
""pleurc; it is therefore continuous with the general
^ce, part of which afterwards becomes the plouro-
P*Moneal cavity of the body, shaded with dots in
^re 9 and marked {pp). It ia thus possible to
from the cavity between the two limbs of ewh.

fold of tlie amnion into the cavity which surtouniii
the alimentary canal. When the several folds meet
and coalesce together above the embryo, they umts
in such a way that all their inner limbs go to form »
continuous inner membrane or sac, and aJl their outd
limbs a similarly continuous outer membrane or 6K
The inner membrane thus built up forms a corapletelf
closed sac round the body of the embryo, and is called
the amniotic sac, or amnion proper (Fig. 9, H, I, &c. o.)i
and the fluid which it afterwards contains is called
the amniotic fluid, or liquoi- amnii. The space between
the inner and outer sac, being formed by the unitai
cavities of the several folds, is, from the mode of iW
formation, simply a part of the general cavity found
everywhere between somatopleure and splanchnopleure.
The outer sac over the embryo lies close under the
vitelline membrane, while its peripheiy is gradually
extended over the yolk as the somatopleuric invest-
ment of the yolk-aac described in the preceding parft-
graph. It constitutes the false amnion while the me
brane of which it forms a part is frequently known
the serous membrane.

The AUantois. If the mode of origin of these two
sacs (Uie inner or true amnion, and the outer or false
amnion, as Baer called it) and their relations to the
embryo be borne in mind, the reader will have no diffi-
culty in understanding the course taken in its growth
by an important organ, the allantois, of which we shall
hereafter have to speak more in detail.

The allantois is essentially a diverticulum of the
alimentary tract, into which it opens immediately in
front of the anus. It at first (Fig. 11, al) fonns a

B] THE ALLANTOIS. 47

flsttened boc projecting into the pleuroperitoneal cavity, ,
thewallsoftbesac being formed of a layer of splancbnie 1
Uesoblast lined by hypoblast.

tit grows forwards in the peritoneal cavity until it |
ihes the Btalk connecting the embryo with the yolk-
and thence very rapidly pushes its way into the space ■
between the true and false amniotic sacs (Fig. 9, G, K).
Curving over the embryo, it comes to lie above the 1
embryo and the amnion proper, separated from the 1
shell (and vitelline mombrane) by nothing more than j
the thin false amnion. In this position it becomeB ^
kighlj vascular, and performs the functions of a respi-
y orgsiL It is evident that though now placed |
te outside the embryo, the space in which it lies is a
tinimtion of that peritoneal cavity in which it took ,
tigia.

t is only necessary to add, that the serous mem-
s, including the false amnioii, either coalesces with I
t Titelline membrane, in contact with which it lies,

e replaces it ; and in the later days of incubation j
f called bj the older embryologista the chorion—
le however which we ahall not adopt.

CHAPTER ni.

THE CHAliaES WHICH TAKE PLACE DURING THE YIBSI
DAY OF INCUBATION.

DuRiNQ the descent of the egg along the oviduct,
where it is exposed to a temperature of ahout 40° C, th»
gennina] disc, as we have seen, undergoea important
changes. Wlien the egg is laid and becomes cold these
changes all but entirely cenae, and the blastoderm
remains inactive until, under the influence of the higbei
temperature of natural or artificial incubation, the vitft
activities of the germ are brought back into play, thi
arrested changes go on again, and usher in the series d
events which we have now to describe in detail.

The condition of the blastoderm at the time whe
the egg is laid is not exactly the same in all eggs ; i
some the changes being larther advanced than in other)
though the dififerencos of course are slight. In aom
eggs, especially in warm weather, changes of the a
kind as those caused by actual incubation may tab
place, to a certain extent, in the interval betwea
laying and incubation ; lastly, in all eggs, both unda
natural and especially under artificial incubation, U

CHAP. UL] the EMBBTONIC SHIELD. 49

dates of the seyeral changes are, within the limits of
some hours, very uncertain, particularly in the first few
days ; one egg being found, for example, at 36 hours in
the same stage as another at 24 or 30 hours, or a third
at 40 or 48 hours. When we speak therefore of any
event as taking place at any given hour or part of any
given day, we are to be understood as meaning that
such an event will generally be found to have taken
place at about that time. We introduce exact dates
for the convenience of description.

The changes which take place during the first day
will be most easily considered under several periods.

From the 1st to about the 8th hour.— During this

period the blastoderm, when viewed from above, is
found to have increased in size. The pellucid area,
which at the best is but obscurely marked in the umn-
cubated egg, becomes very distinct (the central opacity
having disappeared), and contrasts strongly with the
opaque area, which has even still more increased both
in distinctness and size.

For the first few hours both the pellucid and opaque
areas remain approximately circular, and the most im-
portant change, besides increase in size and greater
distinctness which can be observed in them, is a slight
ill-defined opacity or loss of transparency, which makes
its appearance in the hinder half of the pellucid area.
This is known as the embryonic shield.

Slight as are the changes which can at this stage be
seen firom surface views, sections taken firom hardened
specimens bring to light many most important changes
in the nature and arrangement of the constituent
oeDflb

THE FIBST DAY.

[CHA1\

Section op a Bl48todebm op a Fowl's Eoe

AT THt; COUMBNC&UBNT OF iNODBATIOlf.

The thin but complete upper "layer «p
composed of coliuiiiiar cells reals on the in-
complete lower layer I, composed of larger
and more granular cells. The lower layer is
thicker in aomo places than in othara, and ia
especially thick at the periphery. The line
below the under layer marks the upper sur-
face of the white yolk. The larger so-called
formative colls are seen at b, lying on the
white yolk. The tigure does not take in quite
the whole liceadth of the blastoderm; but the
reader must uuderstand that both to the right
band and the left ep ia continued farther than
I, BO that at the extreme edge it rests directly
on the white yolk.

It will b» remembered that the
biastoderui in the uiiiticubated egg is
composed of two layers, ao upper (Fig.
12, ep) and an under layer; that the
upper is a coherent membrano of colum-
nar nucleated ceils, but that the lower
one (Fig. 12, 1) is foi-med of an irregular
network of larger cells in which the
nuclei aru with difficulty visible; and
that in addition to this there ore certain
still larger cells, called 'formative cells'
(Fig. 12, 6), lying at the bottom of the
8egmentation-ca vi ty .

Under the influence of incubatiOT
changes take place very rapidly, which

fUL] THE HYPOBLAST. 51

I leaoltia the formation of the three layers of the blasto-
I derm.

The upper layer, which ia the epihlast already
ftipoken, of (Fig. 13), takes at first but little ehare in

■ thece changes.

In the lower layer, however, certain of the cells

■ bepn to get flattened horizontally, their granules become

■ bss Dumerous, and the nucleus becomes distinct; the
I ttUa 80 altered cohere together and form a membrane.
I The membrane thus formed, which ia first completed in

^ epibkst is represented somewhat diagram mati calif . The
« shew the points of juaction of the two halves of the
The hypoblast is already conatituted aa a momhrane of
d cells, aiid a number of scattered cells are neen between
Pud the epiblAfit.

* Centre of the pellucid area, constitutes the hypoblast.
fclween the hypoblastic membrane and the epiblast
n remain a number of scattered cells (Fig. 13} which
Miot however be said to form a definite layer altogether
Jnct from the hypoblast. They are almost entirely
d to the posterior part of the area pellucida, and

52 THE FIRST DAY. [CSAP.

give rise to the opacity of that part, which we have
spoken of as the embryonic shield.

At the edge of the area, pellucida the hypoblast
becomes continuous with a thickened rim of material,
underlying the epiblaat, and derived from the original
thickened edge of the bla,Btoderm and the subjacent
yolk. It is mainly formed of yolk granules, with a
varying number of cells and nuclei imbedded in it. It
is known as the germinal wall, and is spoken of more in
detail on pp. 65 and 66.

The epiblast is the llomblaU (oomeal layer), and the hjpo-
bkst the DarmdriUenblatt (epithelial glaudtilar l&yer) of the
Oermans, while those parts of the meBnbliut which take part in
the fornuition of the aomatopleiu^ and splauchnopleuro oor-
reapond respectively to the Saut-mii4kBl-plcUle and Darm~/a»er-
ptatle.

All blood-Teasels arise in the mesoblost. Hence the vaaoiUar
layer of the older writers falls entirely within the mesoblnst

The leroim layer of the old authors includes the whole of
the epiblaat, hut also comprises a certain portion of mesoblaat ;
for they speak of all the organs of animal hfe (skiii, bones,
muscle, &c.) as being fonned out ot the serous layer, whereas the
epiblast proper givoH rise only to the epidermis and to certAJn
parte of the nervous system. In the same way their miicrmt layar
ccrresponds to the hypoblast with so much of the mesoblaat aa
takes part in the formation of the orgnns of organic life. Their
veueular layer thereforo answers to a part only of the mesoblaat
viz. that part ia which blood-veasels are ospocially developed.

From tlie 8th to the 12th hour. The changes

which next take place result in the complete differen-
tiation of the embryonic layers, a process which is inti-
mately connected with the formation of a structure known
ae the primitive streak. The full meaning of the

THE PEIMITIVE STREAK.

latter stractore, and its relation to the embryo, can how-
erer only be understood bycompaxiaon with the develop-
ment of the lower forma of vertebrate life.

It will be remembered that in surface views of the
naineubated btaatoderm a small arc, at what we stated
to be the posterior end, close to the junction between
the area opaca and the area pellucida is distinguished
by its more opaque appearance. In the surface view
the primitive streak appears as a linear opacity, which
grsdually grows forwards from the middle of this arc
^ it reaches about one-third of the diameter of the

K4 ^ILLDCIDA Of A VESI -KODSQ BUAStOVK&X OT A ChIOK,

BHEwrao THE Primitive SiaBAK sbortly apibb fib

naST APPKABAKCE.
P'-i. priioitLve streak ; ap. area pelluciiU ; a.oii. urea opaca.

* pellucida. During the formation of the primitive
t the embryonic shield grows lainter and finally
'••jifihes. When definitely established the primitive
'"^ak has the appearance diagrammatically represented

54 THE FIRST DAY. [uHAP.

Sections at this stt^ throw a very important light
on the nature and mode of origin of the primitive
streak. In the region in front of it the bkatoderm is
still formed of two layers only, but in the region of the
streak itself the structure of the blastoderm is greatlj
altered. The most important features in it are repre-
sented in Fig. 15. This figure shews that the median

FERBNriATiON OF THE pKiMiTivB Streak.
The nection paasea through altout tho middle of the primitiTc

pvi. primitive Btreak ; fp. epiblast ; h/, hypjblaat ; i/t, yolk of
the germinal wall.

portion of the blastoderm has become very much thick-
ened (thus producing the opacity of tho primitive streak),
and that this thickening is caused by a proliferation of
rounded cells from the opiblast. In the very young
primitive streak, of which Fig. 15 is a section, the rounded
cells are still continuous throughout with the epiblast, but
they form nevertheless the rudiment of the greater part
of a sheet of mesoblast, which will soon arise in this
repon.

Bl] the primitive streak.

In addition to the cells clearly derived from the
qBblast, there are certain other celb (Fig. 15), closely
^joiniug the hypoblast; these are derivatives of the
interposed between the epiblast and hypoblast,
1 gave rise to the appearance of the embryoi
I daring the previous stage. In our opinion these
&1h) have a share in forming the future :

II thus appears that the primitive sti'eak is essen-
lly a linear proliferation of epiblast cells; the cells
duced being destined to give rise to the mesoblast.
is proliferation first commences at the hinder end of
area pellucida, and thence proceeds forwards.
While the primitive streak is being established, the
blast becomes two or more rows of cells deep in the
IgKiQ of the area pellucida.
Soon after this, the hitherto circular pellucid area '

oval (the opaque area remaining circular). The

13, with remarkable regularity, so placed that its

; axis forms a right angle, or very nearly a right

le, with the long axis of the egg itself. Its narrow

corresponds with the future hind end of the embryo.

" *Kg ^ placed with its broad end to the right hand

I the observer, the head of the embryo will in nearly

cases be found pointing away from him.

The 12th to the 16th hour. The primitive streak

titafiret appearance is shadowy and ill-defined; gradu-

JJ however it becomes more distinct; and during the

period the pellucid area rapidly increases in size,

rffrombeingoval becomes pear-shaped (Fig. 16). The

ifaaitiYe streak grows even more rapidly than the

area; so that by the 16th hour it is not only

66 THK FIRST DAY. [CHAP.

abaolutely, but also relatively to the poUucid area.
longer tlian it was at the 12th hour.

It finally occupies about two-thirds of the length of
the area pellucida; but its hinder end in many instances
appears to stop short of the posterior border of the
area pellucida (Fig. 16). The median line of the

pr. primiiiTe streiLk with primitii
af. amniotic fold
The darker shading round the primitive streak shews the
extenaion of themesol'lBat.

prinutive streak becomes marked by a shallow furrow
miming along its axis. In fresh specimens, viewed with
transmitted light, this furrow appears as a linear trans-
parency, but in hardened specimens seen under reflected
light may be distinctly recognized as a narrow groove.

the bottom of which, being thinner than the sides,
appears more transparent when viewed with transmitted
%hL It is known as the primitive groove. lU depth

■i the extent of its development are subject to great
miatioiis.

During these changes in external appearance then;
grow from the edges of the cord of cells constituting the
primitJTe streak two lateral wings of mesoblast cells,
which gradually extend till they reach the ades of the

1 pellucida (Fig. 17). The two wings of mesoblast
flieet aiong the line of the primitive streak, where they
*ffllreiiiain attached to the epiblast. During this period
oany sections through the primitive streak give an
DBpresaon of the mesoblast being involuted along the lipa
* a groove. The hypoblast below the primitive streak
1* always quite independent of the mesoblast above,
*fiongh much more closely attached to it in the median
Me than at the aides, The part of the mesoblast, which
'6 believe to be derived from the primitive lower layer
" "a, can generally be distinctly traced. In many cases,
Specially at the front end of tho primitive streak, it
"fttia.aa in Fig. 17, a distinct layer of stellate cells, quite
'"like the rounded cells of the mesoblastic involution
« the primitive streak.

In the region in front of the primitive streak, where
™e first trace of the embryo will shortly appear, the
"JWB at first undergo no important changes, except
™st the hypoblast becomes somewhat thicker. Soon,
™*ever, as shewn in longitudinal section lu Fig. 18, the
Ifpoblast along the axial line becomes continuous be-
^^^"5 with the front eud of the primitive streak. Thus
t this point, which is the future hind end of the

1-L

Losghudixal SBcncxs TSBocisa tsz Attji Lckx or
PUMmrx SixEAK. A3r:> tsz Pas? or ihx Kla«tv^
a Fboot or n; or rsx Bla5?od£ui or a Csks. sein-
VHAT Tor:rGXK ZHAS F:«u 19l

pr^ primithne stmk ; ip. cpibfabst : a v. krpobfass cf recxn in
front of pfimitifv okreak ; m. dtx^ ; jrfL Ttjfk of ^eRninftl
wall

embryo, the mesoblast, the epiblast, and the hypobUst
all unite together.

From the 16ih to the 80th hoon. At about the

16th hour, in blastodenns of the stagje lepiesented in
Fig.l6,anmiportantchange takes place in the constitution
of the primitive hypoblast in front of the primitive streak.
The roimded cells, of which it is at first composed (Fig.
18), break up into (1) a layer formed of a single n>w of
more or less flattened elements below-*— the hypoblast
proper — and (2) into a layer formed of several rows of
stellate elements, between the hypoblast and the epiblast
— ^the mesoblast (Fig. 19 m). A separation between these
two layers is at first hardly apparent, and before it has
become at all well marked; especially in the median lino,
an axial opaque line makes its appearance in surface
views, continued forwards from the front end of the
primitive streak, but stopping short at a semicircular

THE FIRST DAY.
Fia. 19.

[chap.

jC/'^" h' fi'TSTWTi^

TKABSvEnsB Section thboitge thh Ebbetosio Bboion op
THE Blabtodhbu ot A Chick hhorxlt prior to the
Formation of tsb Mxciillart Ohoovb aho Moto-

ffl. median line of the eoctiou ; ep. epiblast ; 1.1. lower taj'er oellu
(primitive hypoblast) not yet completely differentifttod into
mesoblaat and hypoblast ; n. nuclei.

fold — the future head-fold — near the front end of the
area pellucida. lu section (Fig. 20) this opaque line is
seen to be due to a special concentration of cells in the
form of a cord. This cord is the commencement of an
extremely important structure found in all vertebrate
embryos, which ia known a^ the notochord (cA). In most
instances the commencing notochord remains attached
to the hypoblast, after the mesoblaat has at the sides
become quite detached (vide Fig. 20), but in other caaea.
the notochord appears to become differentiated in the
already separated layer of mesoblast. In all cases the
notochord and the hypoblast below it unite with tkefrmU
end of tlie primitive streak; with which also the two
lateral plates of mesohlast become continuous.

From what has just been said it ia clear that in tha
region of the embryo the mesohlast originates as two'
lateral plates split off from the primitive hypoblast,

iil]

THE NOTOCHOBD.
Fia. 20.

Tbansvebse Section through the Embrtonic Region of the
Blastoderm of a Chick at the time of the formation
of the Notochord, but before the appearanoe of
THE Medullary Groove.

ep, epiblast ; hy, hypoblast ; cK. notochord ; me, mesoblast ;
yk, yolk of germinal walL

Fia. 21.

Transverse Section of a Blastoderm incubated for

18 HOURS.

The section passes through the medullary groove mc.y at some
distance behind its front end.

A. Epiblast. B. Mesoblast. C. Hypoblast.

m.c medullary groove ; m./. medullary fold ; cA. notochord.

62 THG FmST DAY. [CHAP.

that the notochord origiQatcs simultaneously with the
mesoblast,with which it is at first continuous, as a median
plate similarly of hypobiastic origin.

KOililtBr' holds that the meaohlast of the region of the em-
btTO is derived from a forward growth from thu primitive atre&k.
There is no theoretical objection to this view, and we think it wtmld
be iniposBible to' ehevi for certain hy sectiona whether or no
there is a growth such aa he describes ; but such sections as that
represented in Fig. 19 {and we have seriea of such aectioos from
several embryos) appear to us to be conclusive in favour of the
view that the mesoblaat of the region of the embrjo is to a large
extent derived troin a differeutiatiou of the primitive h^pobltist.
The meeoblaat of the primitive streak forma in part the vascular
atructurcB found in the area pellucida, and probably also in part
the meaobloat of tho allautois.

The differentiation of the embryo may be said to
commence with the formation of the notochord and the
lateral plates of mesoblast. Very shortly after the for-
mation of these parts, tho axial port of the epiblast
above the notochord and in front of the primitive streak,
being here somewhat thicker than in the lateral parts,
becomes differentiated into a distinct medullary plate, the
sides of which form two folds known as the medullary
folds, enclosing between them a groove known aa the
medullary groove. The medullary plate itself consti-
tutea that portion of the ejliblast which gives rise to the
central nervous system.

Between the 18th to the 20th hour the medullary
groove, with its medullary folds or iamiuse dorsales, is
fully established. It then presents the appearance, to-
wards the hinder extremity of the embryo, of a shallow

' Entaick. d. McTuchen u. Jiiilierta Thien: Ldpzig, 1879.

THK NOTOCHORD. - "

gTDove with sloping diverging walla, which embrace be-

t^ween them the front end of the primitiTe streak.

Passing forwards towards what will become the head

of the embryo the groove becomes narrower and deeper

"With steeper walls. On reaching the head-fold (Fig. 22),

^tich continually becomes more and more prominent,

the medullary folds curve round and meet each other in

the middle line, so us to form a somewhat rounded end

to the groove. In front therefore the canal does not

become lost by the gradual flattening and diveigence of

its walls, as is the case behind, but has a definite termi-

nataou, the limit being marked by the bead-fold.

In front of the head-fold, quite out of the region of
tlic medullary folds, there is usually ajiother small fold
fcrmed earlier than the head-fold, which is the begin-
ning of the avmion (Fig. 22).

The appearance of the embryo and its relation to

lie surrounding parts are somewhat diagrammatically

tepreaentcd in Fig. 22, The primitive streak now ends

•ith an anterior swelling (not represented in the figure),

>nd is usually somewhat unsymnietrical. In most cases

■ uis is more nearly continuous with the left, or

J Brely the right, medullary fold than with the medullary

K.|ioaTe. In sections its front end appears as a ridge on

I Bie ride or rarely in the iiiiddlo of the floor of the wide

iBedallary groove.

The general structure of the developing embryo at
ie present stage is best understood from such a section
» that represented in Fig. 21. The medullary groove
0.) lined by thickened t'piblast is seen in the median
le rf the section. Below it is placed the notochord (ch),
Iiich at this stage is a mere rod of cells, and on each

None or the opaque area ib shewn, tbe pear-ahaped ouffi"*
indicating tha limits of the pellucid area.

At the biuder part of the area iBsceu the primitive
pr., with its nearly ptirallel walla, fading away behind, but
ing round and meeting in front so as to form a distinct anteriol
termination to the groove, about halfway up the pellucid aret

Above the primitive groove is seea the medullarj groovs r
with the medullary folds A. These diverging hehind, slope ai
on either side of the primitive groove, while in front they cu
round and meet each other close upon a curved line which rej
eenta the head -fold.

The second curved line in front of and concentric with
first is the commencitig fold of the amnion.

Ill] THE GEBMINAL WALL. 65

side are situated the mesoblastic plates (6). The hypo-
blast forms a continuous and nearly flat layer below.

While the changes just described have been occur-
ring in the area pelludda, the growth of the area opaca
has also progressed actively. The epiblast has greatly
extended itself, and important changes have taken place
in the constitution of the germinal wall already spoken
of

The mesoblast and hypoblast of the area opaca do
not arise by simple extension of the corresponding layers
of the area pellucida ; but the whole of the hypoblast
of the area opaca, and a large portion of the meso-
blast, and possibly even some of the epiblast, take their
origin from the peculiar material which forms the
germinal wall ai^l which is continuous with the hypo-
blast at the edge of the area opaca (vide figs. 15, 17,
18, 19, 20).

The exact nature of this material has been the subject of
many controversies. Into these controversies it is not our purpose
to enter, but subjoined are the results of our own examination.
The germinal wall first consists, as already mentioned, of the
lower cells of the thickened edge of the blastoderm, and of the
subjacent yolk material with nucleL During the period before
the formation of the primitive streak the epiblast appears to
extend itself over the yolk, partly at the expense of the ceUs of
the germinal waD, and possibly even of cells formed around the
nuclei in this part The cells of the germinal wall, which are at
first well separated from the yolk below, become gradually ab-
sorbed in the growth of the hypoblast, and the remaining cells
and yolk then become mingled together, and constitute a com-
pound structure, continuous at its inner border with the hypo-
blast. This structure is the germinal wall usually So described.
It is mainly formed of yolk granules with numerous nuclei, and
a somewhat variable number of rather large cells imbedded

F. & B. 5

66 THE FIRST DAY. [CRtf.

amongst tbem. The nuclei, some of wliioh «re [oiDbabl; endcM
bj a definite cell bodj, tjpicallf form a special lajer ii
atelj below the epiblast. A special moss of Duclei (rid* Figt. IS
and SO, n) is usuallj present at the junctioD of the bjp^dut
with the genniual walL

Tbe germinal wall Petfuns tba characters juat enumerated tOl
□ear tbe close of the first daj of incubation. One function
cells appears to be tbe absorption of yolk material for tbe giowtk
of the embtjo.

The chief events theo of the second period of the
first day are the appearance of the medullary folds
and groove, the formation of the notochord and lateral
plates of mesoblagt, the begiiming of the head-fold and
amnion, and the histological changes taking place in the
several layers.

From the 20th to the 24th hour. A view (
the embryo during this period is given in Fig. '
The head-fold enlarges rapidly, tbe crescentic groove
becoming deeper, while at the same tijne the otcp-
hangicg naargin of the groove (the upper limb of tha
g), rises up above the level of the blastoderm ; in bet
the formation of the head of the embryo may now I
said to have definitely begun.

The medullary folds, increasing in size in &n
dinienaion, but eapecially in height, lean over fron
either side towards the middle line, and thus
more and more to roof in the medullary canal, espe
cially near tbe head. About the end of the first <
they come into direct contact in the region whid
will afterwards become the brain, though they do i
as yet coalesce. In this way a tubular canal is formed
This is the medullary or neurxtl canal (Fig. 23, Fig. H

THE MEDULLARY CASAL,

B3AL View of the hakdbmbd area rEiLUcroA of a Chick
wrm Five Mbsoblabtio SounKs. The Uedollabt
Folds hate met fob farc or thkik extent, but have

T part of the primitive streak ; p./w. posterior part
of tbe primitive streak.

We,). It is not completely closed ia till a period coii-
^erabty later than the one we are considering.

Meanwhile important changes are taking place ip

Be axial port:ion3 of the mesoblast, whifh lie on each

s of tbe notochord beneath thu niedullury folds.

In an embryo of the middle period of this day,

ATniuud with transmitted light, the notochord is

I ai the bottom of the medullary groove between

medullary folds, us a transparent line shining

FOgh the floor of the groove when the embryo is

wed from above. On either side of the notochord

body of the embryo appears somewhat opa^tt*:.

6S THE FIB&rr DAY. [CHlT. H

owing to the thickness of the medullary folds; « H

these folds slope away outwards on either ade^^H

the opacity gradually fades away in the pelludd ani H

Thei^ is present at the sides no sharp line of demaicir ■

tion between the body of the embiyo and the rest of ■

the ai^a: nor will there be any till the lateral foUs ■

make their appearance ; and transyerse yertical sectioos ■

shew (Fig. 21) that there is no break in the mesoblaflk) ■

trom the notochord to the margin of the pellucid aieii ■

but only a gradual thinning. I

During the latter period of the day, however, the I

plates of mesoblast on either side of the notochozd I

begin to be split horizontally into two layers, the QO^ I

of which attaching itself to the epiblast, forms with I

it the somatopleure (shewn for a somewhat later stage I

in Kg. 24). while the other, attaching itself to the I

hypoblast, forms with it the splanchnopleure, Bj I

the separation of these two layers from each other, I

a cs^vity ^Pp\ containing iBuid only, and more con- I

spiouous ill certain parts of the embryo than in otheB» |

:s developer!. This cavity is the beginning of that |

great serous ca^-ity of the body which afterwards becomei

divided into sepciratt* ca\-itits. We shall speak of it as

the pleuro-^ieritoneai caviti/.

rThis cleavage into somatopleure and splanchfio-
pUnm^ oxtends close up to the walls of the medullary
can:U. but close to the medullar}- canal a central or
axijil jK^rtion of eaoh plate becomes marked off by
a sUght .vnstriotiou from the peripheral (Fig. 24), and
rtxvivos tho name of vertebral plate, the more external
mesoblast Uuug caUed the lateral plate. The cavity
l^tweeu the two layers of the lateral plate rapidlj

VERTEBRAL PLATE.

:. while that in the vertebral plat* remains in
p condition of a mere split.

tvSBSB Section thboqoh the Uobsal Rkgion of ak
!]CBRTO OP THE Sbcond Dat (co[iiad from His), iatni-
need bare to illustrate tbe formation of the mesoblostiu
jomitia, and the denvago of the mesoblast
■uduUory ounal ; Pi: mesohlastic somite ; tc, rudiment of
Volffiaa duct; A. epiblast; C. hypoblast; Cli. notoahordi _
J«. Mrta 1 BC. splauchnopleiire. J

At first each vertebral plate ia not only iinbrokeit^
; its length, but also continuoua at it« outer edge
the upper and lower layers of the lateral plate
the same side. Very soon, however, clear trans-
wrae lines are seen, in surface views (Fig. 23), stretch- J
iti([ inwards across each vertebral plate from the edge
if the lateral plate towards the notochord ; while i
raaaparent longitudinal line makes its appearance <
aide of tbe notochord along the line of junction
i the Lateral with the vertebral plate.

The transverse lines are caused by the formatioa |
F vertical cle fts, that is to say, narrow spaces containing J
Dthing but clear fluid ; and sections shew that tlu

THE FIBST DAT.

[iMI^

are due to breaches of continuity in the mraoWaft
only, the epiblast and hypoblast having no share in it
matter.

Thus each vertebral plate appears in surface Ti
to be cut up into a series of square plots, bounded 1
transparent lines {Fig. 23). Each square plot »«
surface of a corresponding cubical mass (Fig. 21, h,
The two such cubical masses first formed, lying one 8
each side of the notochord, beneath and a Uttiel
the outside of the medullary folds, are the fitsl p
of mesoblastic somites'.

The mesoblaslic somites form the basis out ofwU
the voluntary muscles of the trunk and the bodifly
the vertelOTSB are formed.

The first somite rises close to the antericff «
tremity of the primitive streak, but the nezt is b<
to arise in front of this, so that the first-formed i
mite corresponds to the second permanent verteb
The r^on of the embryo in front of the second ftni
Bomit« — at first the largest part of the whole embryo^
the cephalic region (Fig. 23). The somites followingl
second are formed in regular succession from I
backwards, out of the unsegmented mesoblast of '
posterior end of the embryo, which rapidly growi
length to supply the necessary material. With
growth of the embryo the primitive streak is C
tinually carried back, the lengthening of the emb
always taking place between the front end of
primitive streak and the last somite ; and during

' TbeM bodies are
aiDplo; for them the tc

THE NEUttKNTEIUC PASSAGE.

the primitive streak undergoes iinportant

s both in itself and in its relation to the embryo.

b anterior thicker part, which is embraced by the

ing medullary folds, soon becomes distinguished

I iCructure from the posterior part, and is placed

imetrically in relation to the axis of the embryo,

%. 23 ajw); at the same time the medullary folds,

" "i at first simply diverge on each side of the

nitjve streak, bend in again and meet behind so

I completely to enclose tliis front part of the primi-

t streak. The region, where the medullary Iblcb

n as the sinua rhomboidalis of the

) bird, though it has no connection with the

arly named structure in the adult

IHiB u u ooDvenient place to notice remarkable appearances
snt themselves close to the junctioQ of the Deurol plate
jtBUoiiive streak. , These ore temporary passuges leading
•r eiid of the neurnl groove or tube ioto the alimen-
', Tbej vary Bomemhat in different speciea of birds, and
e that in some species there maj bo several openings
I, trbicli appear one after the other and than close again.
• fitet discovered by Oaseer, and are spoken of as the
it pasMgta or canals'. In all cases, with some doubtful
«, thoy lead round the posterior end of the notochord, or
b tltf point where the notochord falls into the primitive

K Hie largest of these passages is present in the embryo duck
>■> tvrenty-sii mesoblostic somites, and is represented in tbo
'« of aections (Fig. 25). The passage leads obliquely back-
fa ud ventralwards from the hind end of the neural tube

' "We Piimitivstreiten bei VagBlembryonan." Sehrift. d. Qrieli.
'• Bijird i, G'ciamnlen SatunoiM$, m Marburg. Vol. n

78 THE FIRST DAY.

Fio. 25.

[chap.

FooR Tranhtebse Sbctionb throcoh the Nbdbksti
Pasbaoe and Adioinii«o Pahts in a DncK Embryo
WITH TwBNTY-sii Mkboblabto Somitbs.

A. Section iu front of the neiirenterio oanal, ahewing a lumoii
ia the notochord.

B. Section through the j;>a38age from the medullary canal
into the notochord.

G. Section shemng the hjpoblaatiu openiag of the neuran-
teric oauaJ, and the groove on the sur&tce of the primitive atreok,
whioh opens in front into the medullary canal.

D. Primitive atreak immediateiy behind the opening of the
neurenteric passage.

mr; medullary canal ; cp. epiblast ; h/. hvpoblast ; cA. noto-
ohoni ; pr. primitive atreak.

THE NEUHENTEKly PAliSAGE. 73

'sto (he notoehod, where the latter joins the priinitivo streak

noiTOiT diverticiiluin from this paasage ia continued for-

•uJi for t, short distance along tlie aiis gf the notoohord (A,

Afier tnvwsiiig the tiotochuriJ, the passage is continiieit

"itua bjpoblastic diverticulum, which opens veatrally into the

*>hn» Jutnen of the alimentary tract (C). Shortlj behind the

"bue the neurenteric paaaage communicates with the

JI^O'il toba the latter structure opens doraally, and a groove ou

■'"tiMor the primitive streak is continued backwards from

"T a short distance (C). The Qnst part of this passage to

■"*' is the hypoblastia diverticulum above mentioned.

■"•■JUTic Lonoitudihal Sbution ihrodoh
**«B End of an EsiBRifo Brno at the t
'^■-JUtiok of the Al1:antijib.
'^l^at; Sp.e. spinal canal ; e/i. notochurd ; n.e. neurenteric
Ay. hypoblast ; p.u.y. poat-aaal gut ; j/r.
itive streak folded in on the ventral side ; c '

last ; an. point where anus will be formed ; p.e.
'***^vijioeral cavity ; am. amnion ; »o. aomatopleure ; up.
•P'^Anclinopleure.

"^ ^te chick we have {bund in some cases an incomplete pBs-
V*iiir to the formntion of the first somite. At a. \is.'wx lAa^

I. aUantois ;

THE FIRST DAY,

[chap.

thrae ia a perforation on the 6oor of the neural coniU, which is
not ao marked aa those in the gooee or duck, and never results
in a complete continuitj between the neural and alimentarj
tracts ; but simply leads from the floor of the neural canal into
the tJBHueB of the tail-awellins, and thenoe into a cavity in
posterior part of the notochiird. The hinder diverticulum of
neural canal along the line of the primitive groove is, moreover,
very considerable iu the clgtck, and is not bo booq obUterated as
in the gooae. The incomplete passage iu the chick arises at a
period when about twelve somitee are present. The tliird
ptLSsage is farmed iu the chick during the tbird day of incub»-

The anterior part of the primitive streak becomea
verted into the tail-swelling ; the groove of the posterior port
gradually shallows and finally disappears. The hinder part itself
atrophies &om behind forwards, and iu the course of the folding
off of the embryo from the yolk the part of the blastoderm where
it waa placed becomea folded in, so as to form part of the ventral
wall of the ombryo. The apparent liiuder part of the primitii
streak is therefore iu re.altty ventral and anterior in relation to
the embryo.

Since the commencement of incubation the i
opaca has been i^preadirig outwards over the surface of the
yolk, and by the end of the first day has reached about
the diameter of a eixjteoce. It appears more or
mottled over the greater part of its extent, but this is
more particularly the case with the portion lying next
to the pellucid area ; so mach so, that around the pel-
lucid area an inner ring of the opaque area majr'%B
distinguished from the rest by the difference of itB
aspect.

The mottled appearance of this inner ring is due to
changes taking place in the mesoblast above the germi-
nal wall — changes which eventually result in the forma-

IIL] SUXMAKT. i9

tion of what is cdDed the weuemlar am, the outer hotder
of which maiks the extreme fimh to whidi the nieeo-
blast extends^

The changes thea whidi occur dnzii^ the fiist day
may thus be biieAy smnmarued :

(1) The hypoblast is fanned as a contiiuioas layer
of plate-like cdb from the lower layer of the segmenta-
tion spheres.

(2) The pcimitiTe streak is formed in the hinder
part of the area peDncida as a linear jxoUferation of
epiblast cellsL These ceUs spread oat as a layer on
^ich side of the pnmitiTe streak, and form part of the
mesoblast.

(3) The primitive grooTe is formed along the axis
of the primitiye streak.

(4) The pellucid area becomes pear-sluqied, the
broad end corresponding with the fiitare head of the
embryo. Its long axis lies at right angles to the long
axis of the ^g.

(5) The medullary plate with the medullary groove
makes its appearance in front of the primitive groove.

(6) The primitive hypoblast in the region of the
medullary plate gives rise to an axial rod of cells forming
the notochord, and to two lateral plates of mesoblast
The innermost stratum of the primitive layer forms the
permanent hypoblast.

(7) The development of the head-fold gives rise
to the first definite appearance of the head.

(8) The medullary folds rise up and meet first in
the region of the mid-brain to form the neural tube.

(9) By the cleavage of the mesoblast, the somato-
pleure separates from the splanchnopleure.

THE FIRST DAY.

[chap. III.

(10) One or more pairs of mesoblastic somites make
their appearance in the vertebral portion of the meso-
blastic plates.

(11) The first trace of the amnion appears in fix>nt
of the head-fold.

(12) The vascular area begins to be distinguished
from the rest of the opaque area.

^^gn CHAPTER IV. ^^'H

TBE CHAMQKS WHICH TAKE PLACE DURING THE FIRST
HALF OF THE SECOND DAY.

General development. ^ attempting to remove
the blastoderm from an egg which has undergone
from 30 to 36 hours' iucubation, the observer can- I

not fail to notice a marked change in the consist- I

ency of the blastodermic structures. The excessive ]

ilclicacy and softness of texture which rendered the 1
extraction of an 18 or 20 hours' blastoderm so difficult, I

has given place to a considerable amount of firmness; I

the outlines of the embryo and its appendages are much I

buMer and more distinct ; and tlie whole biastoderm I

tan be removed from tJie egg with much greater ease. }

In the embryo itself viewed from above one of the
features which first attracts attention is the progress
in the bead-fold (Fig. 27). The upper hmb or head
L ^ua become much more prominent, while the lower
H groove is not only proportionately deeper, but is also
■ being carried back beneath the body of the embryo.
H The medullary folds are closing rapidly. In the
H fegicm of the head they have quite coalesced, a slight
^k^Jl^^y^^middl^in^^he extreme front marking

^^V 7S THE SEC

^H tor some little time theii
^H The open medullary groo^
^^B become converted into a,
^^B in front, but as yet open

ON
li

. a

)

a DAV. -^B

ne of junction (
f the first day
, the neural can

ehind. Even b

H "t \ \

1 "//

■ :dz

V \

/ /

■ --\^

^^B EUBKYO or IBS Chick between t

^^B VIBWED FROM ABOVE A9

^^B (Chromic acid p

^^B f.b. front-brain : mb. mid-brain ; k
^^B cle 1 au.p. auditory jiit ; o.f. \
^^^^^ Bomite; m./. Una of junction ol

mUTT AN-n THIHTT-

iN Opaque Obmw
■eparation.)

1. hind-brwn ; op.v,
■itelline vein ; p.v. I
tbemedulluy^

IV.j

THE BRAIN.

BiedaHarj cana] ; a.r. ainus rhoniboidalia ; (. Uil-folci ; p.r.
'''''*'a8 of primitivo groove (not satia&ictorily repreaented) ;
■V'S^eapeUucida.
!• lins to tie aide between p.v. and m.f. represents the true

"•Wtti of tte embryo,
e nuaJe-gluped outline indicates the margin of the pelluoiii
*f- The head, which reachtis as far back as o./, is dis-
^i^y ttiarked off; but neither the somatopleuric nor
'plaQcbnopleuric folda are shewn in the figure ; the latttr
<*'"fge at the level of o./., the former coa-uderahly nearer
™ frrjat, somewhere between the lines m.b. and h.b. Thi'
*f™ VedclBS op.v. are seen bulging out beneath the superfi-
™ OpibUat The heart lying underneath the opaque body
•^"■^ot be Been. The tail-fold (. ia just indicated ; no dis-
™* lateral folds are as yet visible in the region midway
"^^Oen head and tail. At m./ the line of junction between
tt« medullary folds is stiJl visible, being lost forwards over
™ oawlira! vesicles, while behind may be st-en the remains
'rf'ileanaB rhomboidalia, <.r.

HKliillary folds coalesce completely in the cephalic
'*P°'', the front end of the neural canal dilates into
"■iftli bulb, whose cavity remains continuous vrith
"8 fest of the canal, and whose walls are similarly
"'Bed of «pibla9t. This bulb ia known as the first
*'iroi vesicle. Fig. 27, f.b., and makes its appearance
lie early hours of the second day. From its sides
1 lateral processes almost at once grow out ; they are
>wn as the opHo vesicles (Fig. 27, op. v.), and their
vrill be dealt with at length somewhat latei*
id the 6rst cerebral vesicle a second and a thir>l
make their appearance; they are successively
■ery shortly after the first vesicle ; but the
tion of them may be conveniently reserved to
Uler period. At the level of the hind end of

d to I

THE SECOND DAY.

Flo. 38.

An Embryo CaiCK of auout Thihty-bii Hooaa. vtEWto
FEOM BBLow AB A TttANaPABKNT Object.

FB. tbe fore-brain or firat ccrebnil vesicle, projecting from tfa
Bides of which are aecn the optic vesicles, op. A dofinlt
head in now constituted, the backward Uimt of the aomAtc
pleure fold being indicated b; the faint Ihie S.O.
the head are Heen tbe two limbs of the amniotic head-fgiU
one, the true anmiou a, duaely enveloping the head, t
other, the false amnion a', at eome distance from it. 1!
bead ie seen to project beyond the anterior limit of t
pellucid Hfco.

The spltmchiiopleure folda extend aa far back as *p. AIoDg
diTerging Umba are aeen the conapicuous venous roobi'

JCS. JLsi-zasL. J.X uxml rjiL

&r*sx xadin^enis c-f T£«e: ccziz. if le
^^^ auditory pits i.Tiz- S". sul^ .

*Xlie nninber (^ ziiesciiui&snk sisises

^y ^ ooatznued segi&e&i&dic of Tze tztc

"^««oblasL The fecr or rxe pais iicnei f^rbg -at

^'^^ dav have br 'dkr yniriTr^ jf zhxz s&oizd izt3=a=e£ ^:

*^ xoany &» fifteai. Tie ihSinaic :ak-s ztKe ~ci

*^*<>XTe backvards: ^zki iLr fciiii-rm.:^. ic-r if r-ir =-:ci*r

^^*^^ placed iHadr an a irri wti tLr :»:j:ZiiirT le-

*^^^ii the hind elkl .rf iL- iridt cf il- =iiT= *=*^

^^ &ont end rf the primhfre srr-iai. Fi-r Kinrr — :g

^ aheady formed somhes f :• L^>'; izjirsase iz. siir.

tiiliat at first the embrro cleajiv el-iziri^'i^s it 5*1 r-

^^:^^ to its hinder eni

XmmediatelT behind the lexel cf tbr !as^ i::-=2>>
Sc somite there is jJaoed an enjirreTi.Tr:': •:: tl-=
^^^^losed portiiAi of the meduEarT far:.v. Tlii erlirrr-
^!^^^^t is the sinns rhombcndalis alr%-iy =T«:ker. •:£. It
^ ^ewn in FSg. 23. On its fl>:r i* placed lie frist
^*^^ of the primitive streak. It is a pir^Ij embryoiic
^^^^^^ctore which disappears during the secifiid dav.
^ — In a fonner chapter it was printed oat (p. ST'
Y^^at the emhcyo is YirtnallT formed by a bid
\ P.AB. f

THE SECOND DAT.

[CHAF.

or tucking in of a limited portion of the blastoderm,
first at the anterior extremity, and aftern'ards at the
posterior extremity and at the aides. One of the results
of this doubling up of the blastoderm to foim the head
is the appearance, below the anterior extremity of the
medullary tube, of a short canal, ending bhndly i
front, but open widely behind (Fig. 29, D), a cu/ c!e
aac, in fact, hned with hypoblast and reaching from the
extreme front of the embryo to the point where the
spianchnopleuric leaf of the head-fold (Fig. 20, F. t
turns back on itself. This cul de aac, which of course be-
comes longer and longer the farther back the head-fold is
carried, is the rudiment of the fivnt end of the aliltienf
tary canal, the fore-gut, as it might be called. In trans-
verso section it appears to be flattened horizontally,
and also bent, so as to have its convex surface looking
downwards (Fig, 30, at). At first the anterior end i
quite blind, there being no mouth as yet; the formation
of this at a subsequent date will be described later on.

At the end of tho first half of the second day the
head-fold has not proceeded very far backwards, and'
its hmits can easily be seen in the fresh embryo botk;
from above and from below (Fig, 28).

The heart. It is in the head-fold that the formfr>
' tion of the keuTt takes place, its mode of origin being
connected with that cltavagc of the mesoblast and eon-
sequent formation of splanchnopleure and sonmtopleure
of which we have already spoken.

At the extreme end of the embryo (Fig. 29), where
the blastoderm bt-'gins to be folded back, the mesobUst -
is never cleft, and here consequently there is neither!
somatopleure nor splanchn(>pleure ; but at a point i

last -J
iierfl

TUE yEART.
Fia. »9.

AEiniATlC LOSGITUDIKAI. SECTION THROCGH THE Axis QF

AN Embbto.

Tbcaectiou is supposed to be luade at a time wheu the b<
a lu Mmmeiuwd but the tail-fold has not yd appeared.
iC ostnal cunal, cloacd in froot but as yet ojien behind, Ch.
wtoehwL The section being taken In tbu middle line,
ftu laotovertebra) are of course iint slieivu. In front of
™ nolochord is seen a ntaas of imcloft mesoblnst, which
™i iwutiuilly form part of the akuIL D. the commencing
""gW or front part of tho aiimentary caiiaL F. So.
""""iOfieaTe, t^sed up in it» peripheral portion into the
™ni«tic fold Am. Sp. Splanchnopleure. At Sp. it fonoB
Uunmler wall of tho foregut ; at F. Sp. it in turning round
■"* »bout to ran forward. Jiist at its tiiraiDg pmit the
""'f of the heart Ul. m being developed in its mesoblaat.
PP' pfciinjperitonefll cavity. A epiblast, B niesoblast, C
nJl«ibU»t, iiidicuted In tUo pest of the figure by differences in
Kw «iuiliiig. At the part vrhorc these three lines of reference
^ tlie mnobUst is an yet imclefL

wy Ettle furthei back, close under the blind end of
efwugut. tho cleavage (at the stage of which we are
^^ Dg) be^ns, and the soinatopleiire, F,So, and
^»ndiiiopl«ure, F. Sp. diverge from each ottiei. T\ui^
6—1

thuB enclose bct,w<

THE SECOND DAY, [CHAIV

1 ttem a cavity, pp, which rapidly
increaaes behind by reason of the fact that the fold of
the splanchnopleure is carried on towards the hinder
extremity of the embryo considerably in advance of
that of the Bomatopleure. Both folds, after runmug a.
certain distance towards the hind end of the embryo,
are turned round again, and then course once more for-
wards over the yolk-sac. As they thus return (the
somatopleure having meanwhile given off the fold of
the amnion. Am.), they are united again to form the
uncleft blastodermic investment of the yolk-sac. In
this way the cavity arising from their separation is
closed below.

It is in this cavity, which from its mode of fonn»-
tion the reader will recognise as a part (and indeed at
this epoch it constitutes the greater part) of the general
pleuroperitoneal cavity, that the heart is formed.

This makes its appearance at the under surface and
hind end of the foregut, just where the splanchnopleure
folds turn round to pursue a forward course (Fig- 29,
Ht) ; and by the end of the first half of the second day
{Fig. 28, A) has acquired somewhat the form of a flask
with a slight bend to the right. At its anterior end a
shght swelling marks the future bulbus arteriosus; and
a bulging behind indicates the position of the auricles.
It is hollow, and its cavity opens behind into two
vessels called the viteUine veins (Figs. 27, o.f. and 28 sp.),
which pass outwards in the folds of the splanchno-
pleure at nearly right angles to the axis of the embryo.
The anterior extremity of the hea.rt is connected with
the two aortae.
1 The heart, including both its muscular wall and ita

IV.J THE HEART.

npitbloid lining, is developed out of the aplai>chiii#J
niefoblast on the ventral side of the throat, Bal ^
iiLcc tbe first commence meats of the heart make
their appeantnce prior to the formatioQ of the throat,
the developmeDt of this organ is aomewhal complicated;
uul in order to gain a clear cooception of the manneT
in which it takes place the topography of the re^on
"here it ig formed needs to be very distinctly nnder-
leod.

Intlie region where the heart is about to appear,. I
le splanchnopleure is continuallj being folded in on 1
ilber aide, and these lateral folds are progressively
leeting and uniting in the middle tine to form the under
I teotrul wall of the foregut. At any given moment
e folds will be found to have completely united Jn
■ middle line along a certain distance measured from
« point in front where the cleav^e of the me-sublaat
tbe separation into somatopleure and splanch-
temiB) begins, to a particular point farther back.
!J vUl here be found to be diverging from the point
ire they were united, and not only diverging late-
ij each from the middle line, but also both turning
li to nm in a forward direction to regain the surface
(Ik yolk and rejoin the somatopleure, Fig- 29. In a
BiTene section taken behind this extreme point of
t^ta point of divergence, as we may call it, the
luchnuploure on either side when traced downwards
n tbe axis of the embryo may be seen to bend in
Ihcdsthc middle so as to approach its fellow, and then
I rapidly outwards, Fig. 31. B. A longitudinal
i shews that it runs forwards also at the same
M. Fig. 29. A section through the verj point o£

86 THE SECOND DAY. [cHAP.

diTergence shews the two folds meeting in tlie middle
line and then separating again, go as to form sometliing
like tlie letter x, witli the upper limbs converging, and
the lower limba diverging. In a section taken in
front of the point of divergence, the lower diverging
limbs of the x have disappeared altogether; nothing
is left but the upper limba, which, completely united
in the middle line, form the under-wall of the fore-
gut

Afl development proceeds, what we have called the
point of divergence is continually being carried farther
and farther back, so that the distance between it and
the point where the somatopleure and splanchnopleure
separate from each other in front, i. e. the length of the
for^ut, ia continually increasing.

In the chick, as we have already stated, the heart
commences to be formed in a region where the folds of
the splanchnopleure have not yet united to form the
ventral wall of the throat, and appears in the form of
two thickenings of the mesoblaat of the splanchno-
pleure, along the diverging folds, i.e. along the lower
limbs of the «, just behind the point of divergence.
These thickenings are continued into each other by a
similar thickening of the mesohlast extending through
the point of divergence itself.

Tho heart has thus at first the form of an inverted
V, and consists of two independent cords of splanchnic
mesoblast which meet in front, without however uniting.
As the folding-in of the splanchnopleure is continued
backwarda the two diverging halves of the heart are
.gradually brought together. Thus very soon the develop-
ing heart has the form of an inverted Y, con^sting of an

IT.] THE HEART.

nopiured portion in front and two diverging limba b
bind. Tile unpaired portion is the true heart, while t!
iliw^ng limbs are the vitelline voins already spokei
cf 'Rg. 26, sp). While the changes just spoken of
hsve been taking place in the external form of the
oeart, its internal parts have also become differentiatetl.
A Mvity is formed in each of the halves of the
Mart before even thej have coalesced. Each of these
cavitia iiaa at hrat the form of an irrcgTd:ir space

IRtzbsb Section TUiioi;i;ii lui; I'u.-jtebior Part of tbb

Hkad op a» Eubbto Chick op Thirti Hours.
bind-briun ; rg. vagxis nerve ; tp. cpiblaat ; ch. notochord ;
* thickemng of hypoblMt (possibly a rudiment of the Hub.
■MMhordal rod) ; al. tliroat ; At. heart ; pp. body cavity ;
». widaUc meoobUst ; af. aplanchniu mesablost ; Ay. bv]io-
Uut.

THE SECOND DAY,

[chap.

between the splanchnic mesoLlaat and the wall of the
throat (Fig. 30, /((.). During their fonnation (Fig. 30),
a thin layer of mesoblast remains in contact with the
hypoblast, but connected with the main mass of the
mesoblast of tlie heart by protoplasmic processes. A
second layer next becomes split from the main mass of "J

mesoblast, being still connected with the first layer by the t

above-mentioned protoplasmic processes. These two «:

layers unite to form a tube which constitutes the epilhe- —

lioid lining of the heart ; the lumen of this tube is the ^a
cavity of the heart, and soon loses the protoplasmic c>
trabeculttj which at first traverse it. The cavity of the ^*.
heart may thus be described as being formed by a ^3
hollowing out of the splanchnic mesoblast. Some of the ^^^
central cells of the original thickenings probably become i-=^_^,
blood-corpusclu s.

The thick outer part of the cords of splanchnic meso- — «z3o-
blast which form the heart become the muscular wall^aj^j^
and peritoneal covering of this organ. The musculaK:.4c^^]
wall of each division of the heart has at first the fc
of a half tube widely open on its dorsal aspect, tl
is towards the hypoblast of the gut (Fig. 30 and 31
After th-. -WO halves of the heart" have coalesced in tl»- <ifl
manner already explained, the muscular walls grow
towards the middle line on the dorsal side until th<
meet each other and coalesce, thus forming a comple te
tube as shewn diagrammatical ly in Fig. 31, A. Tb'
remain, however, at first continuous with the splaucho -^f'
mesoblast surrounding the throat, thus forming a pr^^^^^
visional mesentery — the mesocardium — attaching tl^«
heart to the ventral wall of the throat. Tho epithelioi ^
tubes formed in the two halves of the heart remain fW

IV.] THE VASCULAR SYSTEM.

separate, and cause tlie ^vity of the heart to
be divided into two tubes evca after its two halves have
to ill appearance completely coalesced'.

SouQ iift«r its foTToatioti the heart begins to beat ;
it al tirst alow and rare pulsations beginning at the
It and passing on to the arterial end. It is of some
hieteSt to note that its functiooal activity cunimeaceg
hog before the cells of which it ia composed shew any
iistioct differentiation into muscular or nervous ele-
lents,

Vasctllar system. To provide channels for the
laid thiis preBse<i by the contractions of the heart, a
r tubes has made its appearance in tho meso-
>t loth of the embryo itself and of the vascular and
MUucid areas. In &ont the single tube of the bulbus
i bifurcates into two primitive aortw, each
•fwhich bending round the front end of the foregut,
» from its under to its upper side, the two forming
(^Ketlier a sort of incomplete arterial collar imbedded
■ tbe mesoblast of the gut. Arrived at the upper side
^ Ihe gnt, they turn sharply round, and run separate
'"tpuBllel to each other backwards towards the tml, in
ft* mteobla^t on each side of the notochord ii ..nediatcly
■""der the mesoblastic somites (Pigi- 32, Ao, 34, ao).
*bout half way to the hinder extremity each gives ofl"
•right angles to the axis of the embryo a, large branch,
» viUlUne artery (Fig. 36, Of, A.), which, passing
Ktwvds, is distributed over the pellucid aud vascular
a, the main trunk of each aorta passing on with
^^y diminished calibre towards the tall, in whicli it.
n lost.
' This ij not lOiewD in the diafftnm, Fig. 31, ^

90 THE SECOND DAY.

Flo. 31.

hb. hitid bmiii ; ne. notochord ; E. epibUst ; to. somatopletmS
tp. splanchnopleure ; d. alimentary cann] j hy. V
h. (in A) heart ; of. vitelline v
t^In A the two halves of the heart Uave coalesced to fonu ■
d tube Buapended frgm the Toutml wall of the thnwt.

JV J THE VASCCLAR SYSTESL 91

Id D are seen iq the divergiTig folds of the Bplanchnopleure
the tiro Titelline veins {of) which will shortly unite to fonn
tbe ductna vanosufi.

T&AXBTEBSK Section uf as Bubrvo at the emd of tbb
Second Day pasbiko throcgh tru: Rbqion of tee fimsna
dKTERioeua. (Copied from His.)

Jr. medullary canal in tbe region of the hind brain ; F. antarier
cardinal vein ; Ao. Aorta ; Ch. Notochord ; al. alimeatar^'
tanal ; H. Heart (bulbus arteriosus) \ Pp. Pleuroperitoneal

Id the vascular and pellucid areaa, tbe formation of
voecolar channels with a subsequent difTorentiation
into arteries, capillaries and veins, is proceeding rapidly,
Blood -corpuscles Xno are being formed in considerable
numbeis. Tbe mottled yellow vascular area becomes
covered with red patches consisting of aggregations of
blood -corpuscles, oft«n spoken of as blood-islands.

Round the extreme margin of the vascular area and
neaxty completely encircling it, is seen a thin red line,
the tiiMi or vena termimdis {Fig. 36, Sv.}. This will soon
JQcrease In size an<l importance.

From the vascular and pellucid area several large
cbsnnebi are seen to unite and form two large trunks.

' THE SECOND DAY. [CHAP.

one on either side, which running along the splanch-
nopleure folds at nearly right angles to the axis of the
embryo, unite at the " point of divergence " to join the
venous end of the heart, These are the vitelline veins
apoken of above.

Both vessels and corpuscles are tbnned entirely
from the cells of the mesoblast; and iu the regions
where the meBobiast is cleft, are at first observed ex-
clusively in the Bplanchnopleure. Ultimately of course
they are found in the mesoblast everywhere.

In the peUucid area, where the formation of the blood-voasala
lOB-y be most easily observed, a numl>er of mosoblastic cells bib
seen to send out processes (Fig. 33), These processes unite, and
bj thair union a protoplasmio network ia fonued coulaiuiiig
nuclei at the points &om which the prooesses started. The
nuclei, which as a rule are much elongated and contain large oval
uuoleoli, increase very rapidly by division, and thus form groaps
of nuclei at the, so to speak, nodal points of tho network,
Several nuclei may also be seea here and there in the processes
themselves. The network being completed, ti&ae groups, by
contiaued division of the nuclei, iucreaeo rapiSy in size ; the
protoplasm around them acquires a red colour, and the whole
mass breaks up into blood-corpuscles (Fig. 33, b.c.) The proto-
plnam on the outside of each group, as well aa that of the uniting
processes, remains granular, aad together with the nuclei in it
forms the walls of the blood-vessels. A plasma is secreted by
the walls, and in this the blood-corpuscles float &eely.

Each nodal point is thus transformed into a more or leas
rounded mass of blood -corpuscles floating in plasma but en-
veloped by ft layer of nucleated protoplasm, the several groups
t^aiug luiitsd by strands of nucleated protoplasm. These uniting
strands rapidly increase in thickness; new processes are also
continually being formed ; and thua the network ia kept clow
and thickset while the area ia iucreaaing in size.
C By changes similar to those which took place in the nodiil

1
I

IBK VASCULAR SYSTEM.

blood-corpnsclea make their appearance in the pro-
e alw, the centtal portions of which become at the same
llijoBfieil,
fi; the cbDtiDQed widemng of the counecttng processes and
itkm of thor central portions, accompaoied by a corresponding
in the enveloping nucleated cells, the original proto-

Fio. aa.

'ACE Van raoit below op a bmaix i
P<«i»Bi08 End ov thb Pelldcid Akea or * TniBTY-nii
aoi^' CmcK. To il]u«trat« the formation of tht- blood-
^lUries and b!ood-oori>iiBcles, magnified 400 diatnetera.
•B' B)Mil-T<i[pu9cles at <i nodal point, alread; beginning to
■^piw a red colour. They are enclosed in a layer of proto-
V*n, b tlw ontermoBt part of which are found naclei, a.
"^^ trndei nibseqaently become the nuclei of the ceUa
**»S<ig the walla of the TeaaeU. The nodal groups are
"""^d bf protoplafimic proec^aa (p.pr), also containing
"««W with laige nucleoli (»).

TILE SECOND DAY.

plasioic network is converted into a systea

tubes, tlie caoftla of which contain blood-corpuBclea (tod phi

and the walla of which ore formed of flatt«ned nucleated «0|

The blood-corpuscles pass freely from the nodal point*
the hollow processes, and thus the network of protoplim
comes a network of blood-vessela, the nuclei of the corpaaeki
of the walla of irhich hare been, by separate [laths of devdopn
derived from the nuclei of the original protoplasm.

The formation of the corpuscles does not prooeed e^
T^idly or to the same eit«Dt in all parts of the blastodena.
for the greater part are formed in the vascular area, but i
arise in the [lellucid area, especially in the hiuder part. Il
{toal of the pellucid area the processes are longer and the net
accordingly more open ; the corpuscles also are both lab
appearing and less numerous when formed.

Assuming the truth of the alxive account, it is BTident
the blood-vessels of the yolk-sack of the chick do not aiii
spaces or channels between adjacent cells of tlio inesoblHt
are hollowed out in the communicating protoplasmic aubi
of the cells themselves. The larger vessek of the tmnl
however probably formed as spaces betweeu the cells, madi
the case with the heart

r WoUSan duct. Abuut this period there ma;
Been in transverse Bections, taken through the em
(in the region of the seventh to the eleventh sotO
'amall group of cells (Fig. 34, W. d) prujectiug on e
side &om the mass of uncleft mesoblast on the on
of the mesoblastic somites, into the somewhat trianj
space hounded by the epibiast above, the upper
outer angle of the mesoblastic somite on the ii
and the somatic mesoblast on the outside.

Tliis group of cells is the suction of a longitu
ridge, the rudiment of the Wol^aa duct or prin
duct of the excretory system : while the i

le nuMg g|

triim vhich it Bprixigs ia known as the {vtermedieUe
cdl mass. We shall return to tLem immediately.
Siuniliary. The most important changes then which
» place during the firet half of the second day s
I closure of the medullary folds, especially in
'enot part, and the dilatation of the canal so formec

0 the first cerebral vesicle ; the establishmeut of
>bUD namber of mesobliistic somites ; the elevation o

bead from the plane of the blastoderm ; the fom

1 of the tubular heart and of the great blood-vessels;
1 the appearance of the rudiment of the Wolffian |

Itis important to remember that the embryo of vhich -I
are now speaking is simply a part of the whole!
membrane, which is gradually spreading over '
lenu&ee of the yolk. It b important also to bear in
Eld that oil that part of the embryo which is in front
file foremost somite correspouds to the future head,
d the rest to the neck, body and tail During this
riod the head occupies about a tliird of tliL' wholu
ph of the embryo.

CHAPTER V.

THE CHANGES WHICH TAKE PLACE DURING THE
SECOND HALF OF THE SECOND DAY.

One important feature of this stage is the zapid
increase in the process of the folding-ofF of the embryo
from the plane of the germ, and its consequent con-
version into a distinct tubular cavity. At the begin-
ning of the second day, the head alone projected firom
the rest of the germ, the remainder of the embiyo
being simply a jiart of a flat blastoderm, nearly com-
pletely level from the front mesoblastic somite to the hind
edge of the pellucid area. At this epoch, however, a
tail-fold makes its apj^earance, elevating the tail above
the level of the blastoderm in the same way that the
head was elevated. Lateral folds also, one on either
side, soon begin to be very obvious. By the progress
of these, together with the rapid backward extension
of the head-fold and the slower forward extension of
the tail-fold, the body of the embryo becomes more and
more distinctly raised up and marked off from the rest
of the blastoderm.

The medullary canal closes up rapidly. The wide
sinus rhomboidalis becomes a narrow fusiform space.

3
J

id at tie end of this period is ontiroly roofed over, .1
The eonveraion of the original medullary groove into
s closed tube is thus completed.

Ute brain. In the region of the head most im-
portant changes now take place. We saw that at the
legiiming of this day the front end of the medullary
onal waa dilated into a bulb, the first cerebral vesicle,
"liich bj budding off two lateral vesicles became con-
retwi into three vesicles: a median one connected
lyihonlioUow stalks with a lateral one on either side,
lie lateral vesicles known as the optic vesicles (Fig.
?ig. 35, a), become converted into parts of the
(Jt>; the median one still retains the namt: of the first
Wttbral vesicle.

The original vesicle being primarily an involution
rftheepiblast, the walls of all three vesicles are formed
(fepiblast; all three vesicles are in addition covered
sr with the common epiblastic investment which will
Wotually become the epidermis of the skin of the
■•d. Between this superficial epiblast and the invo-
1 epiblast of the vesicles, there exists a certain
intity of mesoblast to serve as the material uut of
^udi will be formed the dermis of the scalp, the skull,
•nd oilier parts of the head. At this epoch, however,
m mesoblast is foimd chiefiy underneath the several
'*^1b8 (Fig. 30;. A small quantity may in section be
m rt the aides ; but at the top the epidermic epiblast
■ titherin close contact with the involuted epiblast of
»e cerebral and optic vesicles or separated from it by
*iid aLne, there being as yet in this region between
•* tifo no cellular elements representing the mesoblast.
The constrictions marking off the optic vesicles also
f.tB. 1

TRAMYEasE Section through thb domal j
Emuhyo op 45 Hooaa.

\ THE BRAIN. 99^

I «pil«st. B. mesoblast. C. hypoblast conaisting of a single
"•^ of flUteoed cells. J/, c. meduUarj cauaL P. v. meBO-
liWic somite. IT. i WoIfBan duct S. o. Somatopleure.
S.p. Splanclmopleure. p.p. pleiiroj>eritoiieal cavity, c A.
""^wlumL a. o. dorsal aorta, v. blood-vessels of the yollt-
*'■ o.p. hne of junction between opaque and pellucid
""*• ; K. |iali3ade<like yolk spheres which constitute the ger-
"linsIwiJI.

™J <'iie-half of the section ia ropMsented in the figure — if
<™pl»tej it Koold be biUlerally symmetrical ubout the liue of
tt«[MiiNllBiy canal.

"6 place of couree beneath the common epiblastic
niTfstiaeiit, which is not involved in them. As a con-
fluence, though easily seen in the transparent

jent fresh J

■" OF A Chick at the Esd of the Seconb Day viewed
rsox &ELOW AS A Trakbpareht Object.
(Copied from Huileyj.

'' fint cerebral vesicle, a. optic vesicle, d. infundibulunu

Tlw ipedmen shews the formatioD of the optic vesicles (a),

' ""itpmrtha from the Ist cerebral veaiole or vesicle of the 3rd

so that the optic vesicles and vesicle of the 3rd ven-

•t flret freely couHnunicatcl with each other, and also the

of the lower wall of the vesicle of the 3rd ventricle into a

the infimcUbuIuni,((0.

1-i

100

THE SECOND DAY.

[chap.

embryo (Fig. 28), they are but slightly indicated in
hardened epecimeoB (Fig. 27).

When an embryo of the early part of the second
day is examined as a transparent object, that portion of
the medullary canal which lies immediately behind the
first cerebral vesicle is seen to be conical in shape, with
its walls thrown into a number of wrinkles. These
wrinkles may vary a good deal in appearance, and shift
&om time to time, but eventually, before the close of
the second day, after the formation of the optical
vesicles, settle down into two constrictions, one separat-
ing the first cerebral vesicle from that part of the
medullary canal which is immediately behind it, ami
the other separating this second portion from a third.
So that instead of there being one cerebral vesicle only,
as at the commencement of the second day, there is now,
in addition to the optic vesicles, a series of three, one
behind the other: a second and third cerebral veaide
have been added to the first (Fig. 27, ?h6, Ai). They
may be also called the " fore brain," the " mid brain,"
and the "hind briiin," for into these parts will they
eventually be developed.

The optic vesicles, lying underneath the epiblast,
towards the end of the day are turned back and pressed
somewhat backwards and downwards against the sides
of the first cerebral vesicle or fore brain, an elongation
of their stalks permitting this movement to take place.
The whole head becomes in consequence somewhat
thicker and rounder.

Before the end of the day the fore brain elongates
anteriorly. The part so established ia not at first sepa-
rate from that behind, but it is nevertheless the first

rn^aind crmwHtfaniaic a rmz i^ ~is 'voorft «(«yi)^
mU> ttc ijomrm m ■milium ■ ; 2iB x> x^ lai; <i&£ «ic m^
dtkT it is jczl! Terr snsikL jb£ =iiA:mccnr!D:<x&

aeweal of Hkt cnssil sisr^c s^^ ^skst JifmuaoKi^
as omgrovtEs c£ t^ X%. ^>, vf rccc cc i2»e sii sai
hind iKUks. \m iktstr S£Tz^:xexss^ ^ie»bfr irhL tktt
of the Sfnal xhcrf«. wH h^ deah wcdi xa ^ aexi

was deacnbei in ibe sccccn i of i&e first dftj, is duiing
the wliole of the seecoi dij s t^ctt coospkuoos ob^vi.
It is seen as a trazsparent rod. somevhas ettipocal in
secdoD (Fig. M, dkr, I]^ii^ immediaiieh* undenieath
the mednllanr canal for the gx«aser pari of its lengthy
and readiing forward in front as fu* as Wlow the
hind bcxder of the first cerebral vesicle*

Cnudal fleznre. Bound the anteriiY tenuination
of the notochord, ihe medoUaiy canal» which up to the
present time has remained perfectly straight^ towards
the end of ihe day begins to curve. The front portion
of ihe canal, tie. the fore-brain with its optic and ct^re-
bral Tesides, becomes slightly bent downwanls, so as to
form a rounded obtuse angle with the r^t of the
«ml»70. This is the commencement of the so-called
cranial flexure and is, mechanically speaking, a con-
sequence of the more rapid growth of Uie doisal wall of
the anterior part of the brain as compared with that of
the ventraL

Anditoiy veside. Lastly, as far as the head is
concerned, the epiblastic plates forming the nidiments oi^
the auditory vesicles become converted into deep p

lOS

THE SECOND DAY.

[chap.

opening one on each side of the hind-brain (Fig. 27,

Heart. We left the heart aa a fuaifonn body
slightly bent to the right, attached to the under wall
of the foregut by the mesocardium. The curvature
iiow increases so much that the heart becomes almost
02 -shaped, the venous portion being drawn up towards
the head so as to lie somewhat above (dorsal to) and
behind the arterial portion, (It would perhaps be more
correct to say that the free intermediate portion is by
its own growth bent downwards, backwards, and some-
what to the right, while the venous root of the heart is
at the same time continually being lengthened by the
carrying back of that " point of divergence " of the
eplanchnopleure folds which marks the union of the
vitelline veins into a single venous trunk.) The heart
then has at this time two bends, the one, the venous
bend, the right-hand curve of the w; the other, the
arterial bend, the left-hand curve of the m. The
venous bend which, as we have said, is placed above
and somewhat behind the arterial bend, becomes marked
by two bulgings, one on either side. These aie the
rudiments of the auricles, or rather of the auricvlar
appmdagea. The ascending limb of the arterial bend
Boon becomes conspicuous as the bulbus arteriosus,
while the rounded point of the bend itself will here-
after grow into the tenti-icles.

VaBcnlar system. The blood-vessels, whose origin
during the first half of this day has been already
described, become during the latter part of the day so
connected as to form a complete system, through which
a definite circulation of the blood is now for the first

v.] THE YASCULAB SYSTEM. 103

time (consequently some little while after the com-
mencement of the heart's pulsation) carried on.

The two primitive aortce haYe already been de-
scribed as encircling the foregut, and then passing
along the body of the embryo immediately beneath
the mesoblastic somites on each side of the notochord.
They are shewn in Figs. 32 A.o. and 34 a,o in section as
two large roimded spaces lined with flattened cells. At
first they run as two distinct canals along the whole
length of the embryo ; but, after a short time, unite at
some little distance behind the head iQto a single trunk,
which lies in the middle line of the body immediately
below the notochord (Fig. 57). Lower down, nearer the
tail, this single primitiYe trunk again divides into two
aortse, which, getting smaller and smaller, are finally
lost in the small blood-vessels of the tail At this
epoch, therefore, there are two aortic arches springing
from the bulbus arteriosus, and uniting above the ali-
mentary canal in the back of the embryo to form the
single dorsal aorta, which travelling backwards in the
median line divides near the tail iQto two main
branches. From each of the two primitive aortse, or
from each of the two branches into which the single
aorta divides, there is given off on either side a large
branch. These have been already spoken of as the
vitelline arteries. At this stage they are so large that
by far the greater part of the blood passing down the
aorta finds its way into them, and a small renmant only
pursues a straight course into the continuations of the
aorta towards the tail.

Each vitelline artery leaving the aorta at nearly
right angles (at a point some little way behind the

104

THE SECOND DAT.

[CHAP.

backward limit of the splanchnopleure fold which is
forming the alimentary canal), runs outwards beneath
the mesoblaatic somites in the lower range of the meso-
blaat, close to the hypoblast. Consequently, when in its
course outwards it reaches the point where the meso-
blast is cleft to form the somatopleure and splanchno-
pleure, it attaches itself to the latter. Travelling along
this, and dividing rapidly into branches, it reaches the
vascular area in whose network of small vessels (and
also to a certain extent in the similar small vessels of
the pellucid area) it finally loses itself.

The terminations of the vitelline arteries in the
/ vascular and pellucid areas are further connected with
Jf the heart in two different ways. From the network of
I capillaries, as we may call them, a number of veins take
I their origin, and finally unite into two main trunks, the
I vitelline veins. These have already been described as
I running along the folds of the splanchnopleure to form
1 the venous roots of the heart. Their course is conse-
' quently more or less parallel to that of the vitelline
arteries, but at some little distance nearer the head,
inasmuch as the arteries run in that part of the splanch-
nopleure which has notyetbeenfoldedin to form the ali-
mentary canal. Besides forming the direct roots of the
vitelline veins, the terminations of the vitelline arteries
in the vascular area are also connected with the sinvs
terminalis spoken of above as running almost completely
round, and forming the outer margin of the vascular
area. This (Fig. 3G, ST.), may be best described as
composed of two semicircular canals, which nearly meet
at points opp(>site the head and opposite the tail, thus all
but encircling the vascular area between them. At the

Y.] THE VASCULAB STSTEM. 105

point opposite the head the end of each semicircle is
connected with vessels (Fig. 36), which run straight in
towards the heart along the fold of the splanchnopleure,
and join the right and left vitelline veins. At the
point opposite the tail there is at this stage no such
definite connection. At the two sides, midway between
their head and tail ends, the two semicircles are espe-
cially connected with the vitelline arteries.

The circulation of the blood then during the latter
half of the second day may be described as follows. The
blood brought by the vitelline veins falls into the
twisted cavity of the heart, and is driven thence through
the bulbus arteriosus and aortic arches into the aorta.
From the aorta, by far the greater part of the blood
flows into the vitelline arteries, only a small renmant
passing on into the caudal terminations. From the
capillary net-work of the vascular and pellucid areas
into which the vitelline arteries discharge their
contents, part of the blood is gathered up at once
into the lateral or direct trunks of the vitelline
veins. Part however goes into the middle region
of each lateral half of the sinus terminalis, and there
divides on each side into two streams. One stream,
and that the larger one, flows in a forward direction
until it reaches the point opposite the head, thence it
returns by the veins spoken of above, straight to the
vitelline trunks. The other stream flows backward,
and becomes lost at the point opposite to the tail
This is the condition of things during the second day;
it becomes considerably changed on the succeeding day.

At the time that the heart first begins to beat the
capillary system of the vascular and pellucid areas is

106

TIIE SECOND DAY.

[chap.

not yet completed; and the fluid whicli is at first driven
by the heart contains, according to most observers, very
few corpuscles.

At the close of the second day the single pair of
aortic arches into which the bulbus arteriosus divides
is found to be accompanied by a second pair, formed
in the same way as the first, and occupying a position a
little behind it Sometimes even a third pair is added.
Of these aortic arches we shall have to speak more fully
later on.

Wolffian duct. During the latter half of the second
day the Wolffian duct to which we have already alluded
becomes fully established, while the first traces of the
embryonic excretory organs or kidneys, known as the
Wolffian bodies, make their appearance. The develop-
ment of the latter will be dealt with in the history of
the third day, but the history of the duct itself may
conveniently be completed here.

The first trace of it is visible in an embryo Chick
with eight somites, as a ridge projecting from the inter-
mediate cell mass towards the epiblast in the region of
the seventh somite. In the course of further develop-
ment it continues to constitute such a ridge as far as
the eleventh somite (Fig. 34 Wd.), but from this point it
grows backwai'ds by the division of its cells, as a free
column in the space between the epiblast and mesoblast.
In an embiyo with fourteen somites of about the
stage represented in fig. 28 a small lumen has appeared
in its middle part, and in front it is connected with
rudimentary Wolffian tubules, which develop in con-
tinuity with it. In the succeeding stages the lumen of
the duct gradually extends backwards and forvardB.

T.j THE AMNION. 107

ud the duct itself also passes inwards relatiTeiy to the
epiblast (fig. 43 lod). Its hind end elongates till it
aimes into connection with, and opens on the fourth
dttj into the cloacal section of the hind-gut.

Tbanmioil and alkntoiB. The amnion, especially
the anterior or head fold, advances in growth very
rapidly during the second day, and at the close of the
dijtompletely covers the head and neck of the embryo;
wmuoh so that it ia necessary to tear or remove it when
the head has to be examined in hardened opaque speci-
mens. The tail and lateral folds of the anmion, though
"ill prc^jressing, Ug considerably behind the head-fold.

The side-folds eventually meet in the median dorsal
<>iie,snd their coalescence proceeds backwards Irom the
Wi-foid in a linear direction, till there is only a small
<i{)emiig left over the tail of the embryo. This finally
Wimes closed early on the third day.

In Figs. 32 and 43 am. the folds of the amnion are
•Wn before they have coalesced. After the coalescence
of the folds of the amnion above the embryo the two
woes of which each is formed become, as already ex-
PwBed in chapter II., separate from each other ; the
•oner, forming a special investment of the embryo, and
"MistitutiLg the amnion proper (Fig, 65), the outer at-
•••^tiiig itself to the vitelline membrane and becoming
"le serous envelope.

The development of the allantois commences during
•M second day, but since it is mainly completed during
"W third day we need not dwell upon it further in this
^ace.

Snmmaiy. The chief events, then, which occur
"Wn^theaecond half of the secood day are as follow:-

108

THE SECOND DAY.

[chap.

1. The Becoad and third cerebral vesicles makt
their appearaoce behind the first.

2. The optic vesicles apring as hollow buda from
the lateral, and the unpaired couunencemeat of the cere-
bral hemispheres from the front, portions of the first
cerebral vesicle.

3. The auditory plate becomes converted into
pit, opening at the side of the hind-brajn or third cere-
bral vesicle.

4. The first indications of the cranial flexure be-
come visible.

5. The head-fold, and especially the splanchiio-
pleure moiety, advances rapidly backwards ; the head of
the embryo is in consequence more definitely formei
The tail -fold also becomes distinct

6. The curvature of the heart increases; ihefint
rudiments of the auricles appear.

7. The circulation of the yolk-sac is established.

8. The amnion grows rapidly, and the allantoii
commences to be formed.

CHAPTER VI.

[K OHAKGES WHICa TAKE PLACE DURING THE TirTRI* ■
DAY.

wil days in tlie history of the chick withiu

this perhaps is the most eventful; the rudi-

l)f so njan; important organs now first make their

D many instances we shall trace the history of these

ks beyond the third day of incubation, in order to
I the reader a complete view of their development.
On opening an egg on the third day the first thing

li attracts notice ia the diminution of the white of
tfg. This seems to be one of the consequences of
Amctiooal acti%'ity of the newly-established vascular
i whose Wood-vease's are engaged either in directly
nrbing the white or, as is more probable, in absorbing
ijolk, which ia in turn replenished at the eicpense of
I vhite. The absorption, once begun, goe.^ on so
irdy that, by the ^nd of the day, the decrease of the

M is very striking.

Tke blastoderm has now spread over about half
lyolk, tbo extreme margin of the opaque ares reach-

110 THE THIRD DAY. [CHAP,

ing about half-way towards the pole of the yolk opposite
to the embryo.

The vascular area, though still increasing, is much
smaller than the total opaque area, being in average-
sized eggs about as large as a florin. Still smaller than
the vascular area is the pellucid area in the centre of
which liea the rapidly growing embryo.

During the third day the vascular area is not
only a means for providing the embryo with nourish-
ment from the yolk, but also, inasmuch as by the dimi-
nution of the white it is brought close under the shell
and therefore folly exposed to the influence of the
atmosphere, serves as the chief organ of respiration.

This in fact is the period at which the vascular area
may be said to be in the stage of its most complete de-
velopment; for though it will afterwards become larger,
it will at the same time become less definite and rela-
tively less important We may therefore, before we
proceed, add a few words to the description of it given
1 the last chapter.

The blood leaving the body of the embryo by the

vitelline arteries (Fig. 36, R Of. A., L. Of. A.) is

carried to the small vessels and capillaries of the vascu-

r area, a small portion only being appropriated by the

pellucid area.

From the vascular area part of the blood returns
directly to the heart by the main lateral trunks of the
vitelline veins, B. Of., L. Of. During the second day

vs venous trunks joined the body of the embryo
considerably in front of, that is, nearer the head thnu,
the corresponding arterial ones. Towards the end of
the third day, owing to the continued lengUiening of

In} THE VASCCLAH AREA

Fia. S6.

nil

t ClBCULATION OP THE YoLK-SaCK AT THE BHP

or THE Third Dat ot Isccbatior.

^ biui A J. the eecond, third and fourth aortic arches ; the
fnt bits become obliterated in its median portion, hut is
Omtinued at its praximaj end aa the external carotid, and at
it« diatal end as the internal carotid. AO. dorsal aorta.
X. Of. J. left viUHioe arteiy. It. Of. A. right vitelline

17. .S*. T. sinus temunalis. L. Of. left vitelline veiD.

Of. right vitelline vein. S. V. aiuua venosua. J). C.
Aldus Cuneri. S. Ca. V. superior cardiiuil orjuguliir vein.
V. Ca. inferior cardinal vein. The veins are marked in

-oa

THE THIRD DAY. [cHAl'.

outline and tbe nrterios are made Mack. The whole blasto-
derm bus been removed from the egg and is supposed to be
viewed from below. Hence the loft is seen on the right, and

the heart, the veins and arteries run not only parallel
to each other, but almost in the same line, the pointe at
which they respectively join and leave the body being
nearly at the same distance from the head.

The rest of the blood brought by the vitelline
arteries finds its way into the lateral portions of the
sinus terminalis, S.T., and there divides on each side
into two streams. Of these, the two which, one on
each side, flow backward, meet at a point about oppo-
site to the tail of the embryo, and are conveyed along a
distinct vein whit-h, running straight forward parallel to
the axis of the embryo, empties itself into the left vitel-
line vein. The two forward streams reaching the gap
in the front part of the sinus terminalis fall into either
one, or in some cases two veins, which run straight
backward parallel to the asis of the embryo, and so
reach the roots of the heart. When one such vein only
is present, it joins the left vitelline trunk; where there
are two they join the left and right vitelline trunks
respectively. The left vein is always considerably
larger than the right; and the latter when present
rapidly gets smaller and speedily disappears.

The chief differences, then, between the peripheral
circulation of the second and of the third day are due
to the greater prominence of the sinus terminalis and
the more complete arrangements for returning the blood
from it to the heart. After this day, although the vas-
cular area will go on increasing in size until it finally

CHANGE OF POSITION OF TUE KMBRTO.

119

D but encompasses the yolk, the promineoce of tbi

BUS toffiUDallB will become less and leas in proportioi

the respiratory work of the vascular area is shifted |

to the alUntois, and its activities confined to absorb-

[iRitritive matter from the yolk.

The folding-in of the embryo makes great pro-

Ras during this day. Both head and tail have become

diatiDct, and the side folds which are to constitute

he lateral walls have advanced so rapidly that the

nbiyo is now a b<md Jide tubular sac, connected with

^ rest of the yolk by a broad stalk. This stalk,

T&seiplained in Chap, il, is double, and consists of

B inner splanchnic stalk continuous with the alimeu-

ny canal, which is now a tube closed at both ends and

a to the stalk along its middle third only, and an

a Eomatic stalk continuous with the body-walls of

embryo, which have not closed nearly to the same

eat as the walls of the alimentary canal. (Compare

"g- 9, A and B, which may be taken as diagrammatic

'^Weotations of longitudinal and transverse sections

m embryo of this period,)

Tie embryo is almost completely covered by the

cion. Early in this day the several amniotic folds

Wl We met and compk-tely coalesced along a line

e back of the embryo in the manner already

''piMntd in the last chapter,

Duiing this day a most remarkable change takes

r*W in the position of the embryo. Up to this

it has been lying symmetrically upon the yolk

the part which will be its mouth directed straight

IWWda. It now turns round so as to lie on its left

P,«B.

L the true oiuniou, very close!; enveloping the li
a only between thu projections of the e
vesicles. It may also be traced at the tail.
In the embryo of whiuh this is a drawing, the heac
1 reached a little fartlier backward than tim ;

YI.] GENERAL VIEW OF EICBRTO. 115

but its limit could not be distinctly seen through the body of the
embryo. The prominence of the &lse amnion at the head is apt
to puzzle the student ; but if he bears in mind the fact, which
could not well be shewn in Fig. 9, that the whole amniotic fold,
both the true and the false limb, is tucked in underneath the
head, the matter will on reflection become intelligible.

C, H, cerebral hemispl^ere. F, B, thalamencephalon or vesicle of
the third ventricle. M. B, mid-brain. H, B. hind-brain. Op,
optic vesicle. Ot otic vesicle. Of V, vitelline veins forming
the venous roots of the heart. The trunk on the right hand
(left trunk when the embryo is viewed in its natural position
from above) receives a large branch, shewn by dotted lines,
coming from the anterior portion of the sinus terminalis.
Ht, the heart, now completely twisted on itself Ao, the
bulbus arteriosus, the three aortic arches being dimly seen
stretching from it across the throat, and uniting into the
aorta, still more dimly seen as a curved dark line running
along the body. The other curved dark line by its side,
ending near the reference y, is the notochord ch.

About opposite the line of reference x the aorta divides into two
trunks, which, running in the line of the somewhat opaque
mesoblastic somites on either side, are not clearly seen.
Their branches however, Ofa, the vitelline arteries, are
conspicuous and are seen to curve round the oommenoing
side folds.

Pv. mesoblastic somites. Below the level of the vitelline arteries
the vertebral plates are but imperfectly cut up into meso-
blastic somites, and lower down still, not at alL

X is placed at the "point of divergence'' of the splanchnopleure
folds. The blind foregut begins here and extends about up
\x) y. X therefore marks the present hind limit of the
splanchnopleure folds. The limit of the more transparent
somatopleure folds m not shewn.

It will be of course imderstood that all the body of the embryo
abofs the level of the reference x, is seen through the portion of
the^yfelk-sac (vascular and pellucid area), which has been removed

8—2

* / - /• #•■»■,

"S". I

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I I

II I
\ I

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''I «'(■.' Ml- I'M viJi lh(.« V« JJi,th«rOIi':Ori

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i| <t ill ii,.lii *>liiili Im ill I 111! wind f^radu-

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■ I It ><mIi ill. I Ititii.i .il |i(iniiittn the whole
I t \ iMt\..l .'11 ih. Il 111 :) sHi^htlv

< I « ..It «

n.] THE BHAIN, llf

tely has the front end of the neural canal
d over the end of the notochord. The com-
ment of this cranial flexure gives the body of an
embtjo of the third day somewhat the appearance of a
leton, the head of the embryo corresponding to the
Wb. On the fourth day the flexure ia still greater
in on the third, but on the fifth and euct-eeding days
it becomes less obvious, owing to the filling up of the
pwti uf the skull

The brain. The vesicle of the cerebral henuBphereB,
•luch on the second day began to grow out from the
front if the fore-brain, increases rapidly in size during
till! third day, growing out laterally, bo as to form two
Wades, so much so that by the end of the day it (Fig.
37, CS, Fig. 3Sj is as large or larger than the original
Wsicle from which it sprang, and forms the most con-
^licuons part of the brain. In its growth it pushes
•Me the optic vesicles, and thus contributes largely to
the roimdness which the head is now acquiring. Each
wenl vesicle possesses a cavity, which afterwards
» one of the lateral ventricles. These cavities are
8 behind with the cavity of the fore-brain.
Owing to the development of the cerebral vesicle the
"^inal fore-brain no longer occupies the front position
(^. 37. FB, Fig. 38, lb), and ceases to be the con-
la object that it was. Inasmuch as its walla will
BT be developed into the parts surrounding the
•Mailed third ventricle of the brain, we shall hence-
™*»nJ ^feak of it as the vehicle of the third ventricle,
U uuluneucephalon.

On the summit of the thalamencepbalon thi
* be aeen a small conical projection, the rudiment

may I

ntof J

. 118 THE THIRD DAY

Fia. S8.

[chap.

Head of a Cbick of thk Third Day viewed sidewaiis ab a

Tranbparent Object. (From Huxley.)
tn. the veaicle of the cerebral hemisphere. lb. the vesicle of
the third ventricle (the ori^nal fore-brain) ; at its sununit
is Been the projection of the jiineal gland e.
Below this portion of the brain is seen, in optical section, the
I optia veaiole a already involuted with its thick inner and tbiuner
[ outer wall (the latter u is placed on the junction of the two, the
primar; cavity being almost obliterated). In the centre of the
Toaicle lies the lena, the abaded portion being the expression of
its cavity. Below th« lens between the two limbs of the horae-
I ihoe ia the choroidal lissure.

II. the mid-brain. III. the hiad-braiu. V, the nidimenta of
the fifth cranial nerve, VII. of the seventh. Below the aeveuth
nerve ia seen the imditory veaicle b. The head having been
mibJBOtod to pressure, the vesicle appears somewhat distorted as
if squeeied out of place. The orifice is not yet quite closed up.

I, the inferior maxillarj process of the first visceral or man-
dibular fold. Below, and to the right of this, is seen the first
visceral cleft, below that again the second visceral fold (2), and
lower down the third (3) and fourth (4) visceral folds. In front
of the folds (i.e. to the left; is aeeu the arterial end of the heart,
the aortic orchea being buried in their respective visceral folds.

/. represents the mesoblost of the base of the brain and spinal
ooid.

THE PITUITARY BODT. 119

(he pineal gland (Fig. 38, e), while the centre of the
floor b produced into a funnel-shaped process, the inf^m-
■iiWuDi {Fig. 39, In), which, stretching towards the

^'•'iniiHAL Section in hough the Brain of a todhq

pRisTiDROa Embbto.
Mraineaoemeot of cerebral hemisphere ; pn. pineal gland ;
A. iafiindjbulum ; pt. ingrowth of moutli to form the
P>taiUi7 body ; mh. mid-brain ; A. corebelhim ; ch. noto-
oborf [ al. alimentary tract ; laa. artery of mandibular arch,
^fteme end of the oral mvsgination or stomodceum,
ft diverticulum of this which becomes the ptfuttory

tlw derelopment nf the pituitary Inidy or hypophysis cerebri
' Iwn the subject of conBiderable controversy amongst embryo-
WB( uid it ia only within the last few years that its origin
'" Uk cml epithelium has been salisfiu.'torily eBtablished.
In the course of cranial flexure tba epiblaat on the under aide
^ head becomes tucked in between the Mind end of the
"Wind the base of the brain. The part so tucked in constitute-s
*™d of bay, and formn the Btomodieum or primitive buccal
ftlroady apofcen of. The blind end of this bay becomes
as a papiUiform diverticulum which may be called the
diverticulum. It is represented as it appeiUB in a

■ liO

THE THIRD DAY. [CHAP.

lower vertebrate embiyo (Elflsmobrancb) in Fig. 39, but is in all
important respects eiaotly similar in the chick. Very abortly after
the pituitary divertioulum becomea first eetablisbed the boundary
wall between the Btomodtcum and the throat becomes perforated,
and the limita of the Htomodraum obbterated, bo that the pituitary
diverticulum looks as if it had arisen from the hypoblast. During
the third day of incubation the front part of the notochord
becomea bent downward, and, ending in a. somewhat enlarged
extremity, comes in contact with the termination of the pituitary
divertioulum. The mesoblaat around increases and grows up, in
front of the notochord and behind the vesicle of the third
ventricle, to form the posterior cliaoid proceaa. The base of tie
vesicle of the third ventricle at the same time grows downwarda
towards the pituitary diverticulum, and forms what ia known as the
infundibulum. On the fourth day the mesoblastic tissue around
the notochord increases in quantity, and the end of the notochord,
though still bent downwards, recedes a tittle from the termination
of the pituitary diverticidum, which is still a triangular space with
a wide opening int^ the alimentary canal.

On the fifth day, the opening of the pituitary diverticulum
into the alimentary canal baa become narrowed, and around the
whole diverticulum an investment of mesoblaat-cells has appeared.
Behind it the olinoid process has become cartilaginous, while to
the sides and in front it ia enclosed by the trabecule. At this
stage, in fact, we have a diverticiilum from the alimentary canal
paaaing through the base of akull to the infundibulum.

On the seventh day the communication between the cavity
of the diverticulum and that of the tliroat has become still
narrower. The diverticulum is all hut converted into a vesicle,
and its epiblastio walls have commenced to send out into the
mesoblastic investment soUd processes. The infundibulum now
appeara as a narrow process from the base of the veaicte of tiaa
tiiird ventricle, which approaches, but does not unite with, the
pituitary veaidu.

By the tenth day the opening of the pituitary vesicle into
the throat becomes almost obliterated, and the lumeu of the
vesicle itaelf very much diminished. The body consists of
cords of epiblaat-uells, the niesoblast between

Tl]

THE PITUITAET BODY.

121

wiaob bw tbeadj corameaced to become vasculiu-. The conb
ttm of eptblast c«lk are surrounded bj a delicate mem-
propria, tad a few of them possess a small lumeD. The
itahm bus iDcreaaed in length. The relative positions of

fteptaHaiy body and infundibulum are shewn iu the figure of

tte twill in Chapter vm.

On tl» twelfth day the communication between the pitiiitarj
ncle and the throat is entirely tibiiterated, but a solid cord of

^1» itiil cowiBcfa the two. The veaselB of the pia mater of the
tt the third feDtricle have become connected with the

pl"™!? body, and the infandibulurn has grown down along its

"""tainr border.

lo the later stages all connection ia lost between the pituitary

"^ and the throat, and the former becomes attached to the
^pMprccana infiindibiUi.

i** imI nature of the pituitary body is still eitremely obscure,
' it ia not improbably the remnant of a glandular structure
«1 tuy have opened into the mouth in primitive vertebrate
»^ but which has ceased to have a function in existing

Wl»bat«i.

ond an iacrense in size, which it shares with
^^^J all parte of the embryo, and the change of
"•otion to which we have already referred, the mid-
ondeigoea no great alteration during the third
T' Its roof will ultimately become developed into
Ofiyoro bigemitw. or optic lobes, its floor will form
■* crura cerebri, and its cavity will be reduced to the
■•now canal known sB the iter a terUo ad quartv/m.
•■Wriniiunt,

In the biod-braiQ, or third cerebral vesicle, that
^ which lies nearest to the mid-brain, is during

wilhalin Mlillei Viher iie Enlaieklung und Bau der Hypaphj/tU
' ia Pnettnu InfandiiuU Cerebri. JtnaUche Zeiltchri/i. I
'^ Uil V. Ton UihaUtovios, Whbeliaiti u. HiTnanhang, ArcJiiv f.
^- itt. Vol, II. 187S.

1X2 THE THIBD DAY, [vSH.

the third day marked off from the rest by a sliglil
constriction. This distinction, which becomes mnch
more evident later on by a thickening of the walls and
roof of the front portion, separates the hind-brun inW
the cerebelttmi in front, and the medulla oftfonyaB
behind {Figs. 38 and 39). While the walla of ih*
cerebellar portion of the hind-brain become very oudi
thickened as well at the roof as at the floor and sifi,
the roof of the posterior or medulla oblongata portioo
thins out into a mere membrane, forming a delicsB'
covering to the cavity of the vesicle (Fig. 40, iv), which
here becoming broad and shallow with greatly tMA-
ened floor and sides, is known as the fourth vetdridl,
subsequently overhung by the largely developed p*
teriot portion of the cerebellum.

The third day, therefore, marks the differentiation
of the brain into five distinct parts : the cerebnJ
hemispheres, the central masses round the
ventricle, the corpora bigemina or optic lobes, ih*
cerebellum and the medulla oblongata; the oiipBl^
cavity of the neural canal at thu same time p
from its temporary division of three single cavities intt
the permanent arrangement of a series of connectw
ventricles, viz. the lateral ventricles, the third venttidfi
the iter (with a prolongation into the optic lobe
each side), and the fourth ventricle.

At the same time that the outward external aBl^
of the brain is thus being moulded, internal chaogN
lire taking place in the whole noural canaL These ■
best scon in sections.

At its first formation, the section of the txritf-
the neural canal is round, or nearly so.

THE CRASIAL AND 8PDJAL NERVES.

1-2A

About this time, however, the lining of involuted
(blast along the length of the whole spinal cord
very much thickened at each side, while
ig but little at the mid-points above and below.
n result of this is that the cavity as seen is section
1 6i &nd 65), instead of being circular, has become
anow vertical slit, almost completely tilled in on
3k ade.

Id the region of the brain the thickening of the
J epiblast follows a somewhat different course,
kile almost everywhere the sides anil floor of the
il are greatly thickened, the roof in the region of
Bvahous ventricles, especially of the third and fourth,
I escesKvely thin, so as to form a membrane
iiced to almost a single layer of cells. (Fig. 40, iv.)
Cnuii&l and spinal nerves. A most important
"^t which takes place during the second and third
is the formation of the cranial and spinal nerves.
B within a comparatively recent period embryologists
B nearly unanimous in believing that the peripheral
Tu originated from the mesoblast at the sides of
'^naa and spinal cord. This view has now however
B definitely disproved, and it baa been established
It both the cranial and spinal nerves take their origin
^Wtgrowths of the central nervous system.
The cranial nerves are the first to be developed and
e before the complete closure of the neural groove.
J are formed as paired outgrowths of a continuous
0 known as the neural band, tomposed of two
OB, which connects the dorsal edges of thi
fly dosed neural canal with the estumal fpiblaat.
mode of development will best bt- understood

THS TBIBD DAT.

hfwiliiefa. Tike aMlaoa ahem tlw reiy Uiiu roof «d
v«dMof the nstride.
L SglodMid— (di^nmmatie shadiiig).
3r tokqIat Tciii.
1 aaidH«(7 Tedele. CC pMnta to the end «UiA
cmaL RL. Reccssos Ul> vrintiiL ibt
± Suing tlie alimeiitatj cac&L hg is itself plaoed b
dH csri^ of tfao alimenUiy ouisl, id that part of the «mmI
oUefc win iMoome Um tfaraal. The ventnl (anterior) «al| tf
tfae csbaI >■ not abevo in the aection, bat on each nda ■
Mco poriims of m fair of risoenl udies. la cAob mi
■ aeen the wctton of the acrtic arch JOJ belongii^ to tt
naeeral atch. The tiumcI thus cat through ia r^atM^
Inwards towania the head, being about to join the Anljjfl
atvte AO. Bad the section been neaier the bead^Mj
catned throng the jJaue at which the aortic arch c

] THE CRANIAL AND SPINAL NERVES. 12S'l

"mi the alimeiitAry canal to reach the mesobkBt abofc it,
iOi ui AO would have fcrmod one continuouB curved
1*'% Id Bections lower down in the back the two aortce,
AO, gQe 00 eftch side, would be found fused into one median i

of Fig. 41, where the two roots of the i
wpis Mne {vg) are shewn growing out from the neural
"OA Shortly after this stagu the neural band becomes
•epattted from the external epiblaat, and coastitutes

•WMM SacnoN XHBonGB TBB PoaiERioH Pabt of
Head pr ah Eubryo Chick of Thirty Hours.

■inJ-biBin ; vg. vuguo nerve ; ep. e])iblaat ; ch. notochord ;
f. ttiokening of hypoblast (poasibly a rudiment of the aub- '
'>Btoolu>idal rod) i al. throat ; hi. heart ; pp. body cavity ij
M. mnitic DQeeoblaEt ; af, aplanchnic mesobluit ; hj/. hypo^ I

126 THE TRIKD DAV, [CHAP.

a, crest att.'iched to the roof of the brain, while its two
laminfB become fused.

Anteriorly, the neural creet extends as far as the
roof of the mid-brain. The pairs of nerves which
undoubtedly grow out from it are the fifth pair, the
seventh and auditory {us a single root), the glosso-
pharyngeal and the various elements of the v^us (as a
mngle root).

After the roots of these nerves have become estab-
lished, the crest connecting them becomes partially
obliterated. The roots themselves grow centrifiigally,
and eventually give rise to the whole of each of the
cranial nerves. Each complete root develops a gan-
glionic enlargement near its base, and (with the ex-
ception of the third nerve) is distributed to one of the
visceral arches, of which we shaU say more hereafter.
The primitive attachment of the nerves is to the roof
of the brain, but in most instances this attachment is
replaced by a secondary attachment to the sides or
floor.

The rudiments of four cranial nerves, of which two
lie in front of and two behind the auditory vesicle,
axe easily seen during the third day at the sides of the
hind-brain. They form a series of four small opaque
masses, somewhat pearshaped, with the stalk directed
away from the middle line.

The most anteiior of these is the rudiment of the
fifth nerve (Figs. 42 and 67, V). Its narrowed outer
])ortion or stalk divides into two bands or nerves. Of
these one passing towards the eye terminates at present
in the immediate neighbourhood of that organ. The ■
other branch (the rudiment of the inferior maxillaiy

THE CRAJJIAL NERVES.
Flo. 43.

B Embryo Chick or thk THiao Day (Sbvbmti-
^n HocBs) YiEWED bsewats as a Tbakspabest Object.
[fna Htaky.)
h. ontnl hemiapfaerea. lb. TeaioJe of the third ventricie. IL
Bkj.lwuL lU. hind-bmiii. ^. naeal pit a, optic vesicla.
& otio veeide- d. inliiiidibulum. e. pineal bodj. A. noto-
cbori V. fifth nerve. VII. seventh nerve, VIII. united
ilwopbuTtigeai and pufnimogastrio nerves. I. 3, }. 4, $
Ibe fire itacera] folds.

Wh rf tae fifth nerve) is distributed to the fint
eealarck

The second mass (Pigs. 4-2 and 67, Vll) is the rudi-

nl of the seventh, or facial nerve, and of the audi-

••y nerve. It is the nerve of the second visceral arch.

^e two masses behind the auditory vusiclc repre-

it the glossophan-ngeal and pneiimogaftric nerves

(i%. 42, Vni. Fig! 67, O. Ph. and Pg.). At first

"iW, they subsequently become separate. The glosao-

puyngeal supplies the thlid arch, and the pneumo-

Riic die fmuth and succeeding arches.

Ik UUr devdopment of the cnuual nerivs baa ouly bean
'tUf waited out, and we will oonfioe ooTBdves here to .

J MOD J

THE THIRD DAY. [CHAP.

brief Btatemect of ttome uf the main resulU arrived at. The
outgrowUi for the vagus uun'e supplies iu the embrja the fourth
aad suoceeding visceral arches, aoil from what we know of it
m the lower rertebrate tjpes, we may conclude that it is a
compound nerve, composed of as maiiy priniitively distinct
nerves as there are branches to the visceral arches.

The glossopharyngeal nerve is the nerve supplying the third
risceral arch, the homologue of the lirat branchial arch of Fiahes.
The development of the hypoglossal nerve is not known, but it is
perhaps tbe anterior ruot of a spinal nerve. The spinal acoesaorj
nerve has still smaller claims than the hypoglossal to be regarded
aa a true cranial nerve. The primitively siagle root of the
seventh auditory nerves divides almost at once into two branahea.
The anterior of tbei^e pursues a straight ooiu^e to the hjoid ardh
andfoTlnstlierudimentof the facial nerve, Fig. 67, vn; the second
of the two, whiuh is the rudiment of the auditory nerve, develops
B, ganglionic enlar^ment, and, turning backwards, closely hugs
the ventral wall of the auditoi^ involution. The sixth nerve
appears to arise later than the seventh nerve iroQi the ventral
part of the hind-brain, and has no ganglion near its root.

Shortly after its development the root of the fifth nerve shifts
BO as to be attached about half-way down the side of the brain.
A large ganglion is developed close to the root, which becomes
the Oasserian ganglion. The main branch of the nerve grows
into the mandibular arch [Fig. 67), maintaining tuworils it similar
relations to those of the nerves behind it to their respective
arches.

An important branch becomes early developed which is
directed straight towards the eye (Fig. 67), near which it meets
and unites with the third nerve, where the ciliary ganglion
is developed. This branch is UHually called the ophthalmic
branch of the fifth nerve, and may perhaps represent an inde-
pendent nerve.

Later than these two branches there is developed a third
branch, iwsBiug the upper process of the first visceral arch.
It forms the superior nmxillai'y branch of the adult.

Nothing is known with reference to the development of the
fourth nerve.

Ttl THE SPIMAL NERVES. 12

Tit hiitory of the third uerre is still imperfectly known.
utn it develops early on the aecoad day from the neural
>t*ti 00 llie roof of the Dud-br&in, an outgrowth on each side,
nririiiiiilar to the rndimeat of the posterior nervea. Thia out-
povtli is believed by Mursliall to be Che third nerve, but It must
» htme in mind that there Ib no direct evidence on the point,
tlw ht« of the outgrowth in queatioa Dot having been natisfao-
teii/foUowed

a very conaiderably later period a nerre uay be found
^nyiitg from the floor of the mid-brain, which is undoubtedly
tw liiid nerve. If identical vrith the outgrowth just spoken of,
i mtut have shifted ita attachment from the roof to the floor of
ftebaio.

The nerve when it springs from the floor of the brain runs
Utt^f backwards tiil it terminates in the ciliary ganglion,
n which two branches to the eye-musclea are given off.
[1. Htnhall. "The development of [he cranio! ncrvcB in the
Tli*" 9<wrt. Journal of MicroKop. Seirnet, Vol. iviii.]

Ib the case of the spinal nerves the posterior roots
■iginate as outgrowths of a series of median processes

■ celU, which make their appearance on the dorsal side

■ the spinal cord. The outgrowths, symmetrically
|Ii»d on each side, soon take a pj-riform aspect, and
^J themselves to the walls of the spinal cord, Tliey
R represented as they appear in birds in Fig. 43, sp. g.,
*d as they appear in a lower vertebrate form in Fig. ■it.

The original attachment of the nerve -rudiment to
^^M medullary wall is not permanent. It becomes, in
•ct, very soon either extremely delicate or absolutely
Mernpted.

The nerve-rudiment now becomes divided into three
•t*, (1) a proximal rounded portion ; (2) an enlarge*)
"Mdle portion, forming the rudiment of a ganglion ; (3)
idietaJ portion, forming the commencement of the nerve,
'■'e ptorimal portion may very soon be observed to be

Th&SBVERSB BECnON THROUGH THE TRUNK OF A DuCk EUBRTO
WITH ABOUT TWEMI FOUR MESOBLABTIC SOMITES.

am. amnion ; lo. somntopJuure ; tp. aplnnchnopleure ; ind. Wolffian
duct ; si. segmental tube ; ca.ii. cardinal vein ; mt, muscle-
plate ; ip.ff. spinal ganglion ; ip.c. apinal oord ; c/i, notochord ;
ao. aorta. ; hj/. hypoblast.

united with the side of the spinal cord at a very con-
siderable distance from its original point of origin. It is
moreover attaehed, not by its extremity, but by its aide.
The above points, which are much more easily
studied in some of the lower vertebrate forms than in
Birds, are illustrated by the subjoined section of an
Elasmobranch embryo. Fig. 45.

THE 3PI>'AL XEBTES.
Fio. 41

n SBCnOK TEBOL-i

. KODKU KMBSIO

I M oeuitl anal ; pr, posterior root of spitiAl nerve j *. sub-
""''^^Wkl rixl ; ao. aorta ; tc bomatic mMoblaat ; »p.
^'*wliiuo tneeoblast ; mp. mufloie-plat* ; mp'. portion of
"""ole-plito convertod into muscle ; I'r. portion irf the
'''^bnl plate Nrhich will give riae to the vertebral bodjen ;
<^ ilimantMj tract.

" i8 Bitjemely difficult to decide whether the per-

" attachmeut of the posterior nerve-roota to the

cowl is entirely a new formation, or merely due

we ahifling of the original point of attachment

Me inclined to adopt the former view.

The origin of the anterior roots uf the spinal nerves

not OS yet bt^en satisfactorily made out in Birds ;

It it a])[>earB probable tliat they grow from the ventral

"^onier of the spinal cord, considerably later than the

i^«terior roots, && a number of strands for each nerve,

[chip.

SKmON THROCOn THE DORSAL RBQIOK OF

pr, posterior root ; «f>.^. spinal ganglion ; n. nerve ;

ment of ganglion to spinal cord ; no. neural canal ; mp.
muscle-pkte ; ek. notochord ; i. inveetment of spinal cord.

which subsequently join the posterior roots below the
ganglia. The shape of the root of a completely formed
spinal nerve, as it appears in an embryo of the fourth
day, is represented in Fig. 68.

The Eye, In the preceding chapter we saw how
the first cerebral vesicle, by means of lateral outgrowths
followed by constrictions, gave rise to the optic vesicles.
These and the parts surrounding them undeigo on the
third day changes which result in the formation of the

At their first appearance the optic vesicles stand
out at nearly right angles to the long axis of the
embryo (Fig. 27), and the stalks which connect them

*ith the fore-brain are short and wide. The con-
fticlioBs which give rise to the stalks take place chiefly
fom above downwards, and also eomewhat inwards and
aiwards. Thus from the first the vesicles appear
^liiig from the under part of the tore-brain.

These stalks soon become comparatively narrow,
id constitnte the rudiments of the opti
KJ). The constriction to which tlie stalk or optic

ft)

133

QOK rsBOCOB THE HEAD OP AN EJiBBYo Teleosikui, to

(HtV TKM roKllATlON OF THE orTIC VEalCLEB, ETC. (From

Oegenbaor ; after Scbenk.)
fcn-bRun ; a. opUo vesicle ; b. stalk of optic vesicle ; d.

! is due takes place obliquely downwards and

•cWanis, so that the optic nerves open into the base

I lie front part of the thalamencephalon (Fig, 46 b).

Wliile these changes have been going on in the

0 ildks, development has also proceeded in the

* of the vesicles themselves, and given rise to the

V^iaente of the retina, lens, Titreous humour,

tt part* of the eye.

134

THE THIRD DAY.

[chip.

Towards the end of tlie second day the external
or superficial epiblaat which covers, and is in &U. but
immediate contact with, the most projecting portion of
the optic vesicle, becomes thickened. This thickened
portion is then driven inwards in the form of a shallow
open pit with thick walla (Fig. 47 A, o), carrying beSon
it the front wall (r) of the optic vesicle. To such ai
extent does this involution of the superficial epibUst
take place, that the front wall of the optic vesicle is
pushed close up to the hind wall, and the cavity of the
vesicle becomes almost obliterated (Fig. 47, B).

The bulb of the optic vesicle is thus converted into
a cup with double walls, containing in its cavity the
portion of involuted epiblast, This cup, in order to
distinguish ita cavity from that of the original optic
vesicle, is generally called the secondary optic vesidt.
We may, for the sake of brevity, speak of it as ibe
optic cup; in reality it never is a vesicle, since it
alw#y3 remains widely open in front Of its double
walls the inuer or anterior (Fig, 47 B, r) is fonned
from the front portion, the outer or posterior (Fig. 47
B, u) from the hind portion of the wall of the primaiy
optic vesicle. The inner or anterior (r), which vov
speedily becomes thicker than the other, is converwi
into the retina ; in the outer or posterior {«),
remains thin, pigment is eventually deposited, and i*
ultimately becomes the tesselated pigment-layer of tto
choroid.

By the closure of its mouth the pit of involuW*
epiblast becomes a completely closed sac with tbic*
walls and a small central cavity (Fig. 47 B, I).
the same time it breaks away from the external *

iTio Sections illfiithatiso the Furmaiion urj
THK ESK. (After Bomak.)
^n A, the thin SDperfici&l epibUst h is eeco to be thickened at j^
in fiont of the optic vesicle, and involuted bo as i
a pt o, tlie mouth of which hns nheody begun to cl
Owing to this involution, trbich forma the rudiment of the
kna, the optic vesicle is doubled in, its front portion r being
pnahed agunst the back portion u, and the original oavitj
c< tha Teeicle thus reduced in size. The stalk of the vesicle
ia ibcvD as still broad.

S, tlie optjo vesicle is still further doubled in so as to form a
cop with a posterior ^rall u and an £Uit«rior wall r. In the
Wlow of this cup lies the lens Z, now completely detached
iperficial epiblnst x. Its cavit; is still shewn.
Tbe cavitj of the stalk of the optic vesicle is olreadj much

t, vliich forms a continuous layer in front of it,
Qtnces of the original opening being lost. There is
» left lying in the cup of the secondary optic vesicle,
1 iiolat«<l elliptical mass of epiblast. This is the
the lens. The small cavity within it
|*«lily becomes still less by the thickemiig of the
^ especially of the hinder one.
At its first appearance the lens is in immedinte
VAnA with the anterior -wall of the secondary optic
le (Fig, 47 B). In a short time, however, the lens

136

THE THIRD DAY.

[chap.

is seen to lie in the mouth of the cup (Fig. 50 A), a
space {vh) (which is occupied by the vitreous humour)
making its appearance between the lens and anterior
wall of the vesicle.

In order to understand how this space is developed,
the position of the optic vesicle and the relations of
its stalk must be borne in mind.

The veside Ues at the side of the head, and its
stalk is directed downwards, inwards and backwards.
The stalk in fact slants away from the vesicle. Hence
when the involution of the lens takes place, the direc-
tion in which the front wall of the vesicle is pushed in
is not in a line with the axis of the stalk, as for
simplicity's sake has been represented in the diagram
Fig. 47, but forms an obtuse angle with that axis, after
the manner of Fig. 48, where 8 represents the cavity

Fiu. 48.

DiAORAHMATIC SECTION OF THE EyE AND THE OpTIC NeRVE

AT AN EARLY STAGE (from Lieborkuhn),

bo shew the lens I occupying the whole hollow of the optic cup,
the inclination of the stalk s to the optic cup', and the
continuity of the cavity of the stalk s' with that of the
primary vesicle c ; r, anterior, u posterior wall of the optic
cup.

vl] the eye. 137

of the stalk leading away from the almost obliterated
cavity of the primary vesicle.

Fig. 48 represents the early stage at which the
lens fills the whole cup of the secondary vesicle. The
subsequent state of afiGedrs is brought about through
the growth of the walls of the cup taking place more
rapidly than that of the lens. But this growth or this
dilatation does not take^ place equally in all parts of
the cup. The walls of the cup rise up all round except
that part of the circumference of the cup which
adjoins the stalk. While elsewhere the walls increase
rapidly in height, cairying so to speak .the lens with
them, at this spot, which in the natural position of the
eye is on its under surface, there is no growth: the
wall is here imperfect, and a gap is left. Through this
gap, which afterwards receives the name of the chh
raidal fissure, a way is open from the mesoblastic tissui
surrounding the optic vesicle and stalk into the interior
of the cavity of the cup.

From the manner of its formation the gap or fissure
is evidently in a line with the axis of the optic stalk,
and in order to be seen must be looked for on the
under surface of the optic vesicle. In this position it
is readily recognized in the transparent embryo of the
third day. Figs. 37 and 48.

Bearing in mind these relations of the gap to the
optic stalk, the reader will understand how sections of
the optic vesicle at this stage present very diflFerent
appearances according to the plane in which the
sections are taken.

When the head of the chick is viewed from under-
neath as a transparent object the eye presents very

138

THE THIRD DAT.

[chap.

much tte appearance represented in the diagram
Fig. 49.

A section of auch an eye taken along the line y,
perpendicular to the plane of the paper, would give a
figure corresponding to that of Fig. 50 A. The lens,
the cavity and double walls of the secondary vesicle, and
the remains of the primary cavity, would all be repre-

DiAaauouTic Befrebbntation or the Eie or the CmcK

OF ABOUT TBI THIRD DAT AB SEES WHEIT THE HEAD IB
VIKWKD PBOM UUDEENBATH AB A TBANBPARKKT OBJECT.

I the lens, S the cavity of the lens, lying in the hollow of the

optic cup.
T the anterior, k the poalerior wall of the optic oup, e the cavity

of the primary optic vesicle, now nearly obliterated. By

inadvertence u has been drawn thicker than r, it should

have been thinner throughout.
t the staik of the optie cup with i its cavity, at a lower level

than the cup itaelf and therefore out of focua ; the dotted

hne indicateH the continuity of the cavity of the stalk with

that of the primary vesicle.
The line i, i, through which the section shewn in Fig. 50 C ia

Buppoaad to be taken, passes through the choroidal fisauru.

^^'^FMnmMtin aectioo takeu perfieiiiliouLu' to tbe [ilane of
'^ I»per, along the lino y, y. Fig. 49. The staUc is Dot
■M9, tlie ledion (oiling quite out of its region, vk, hollow
of optic cup filled with vitreous humour ; other letters as in
% W B.

"> Stctimi liken poftllel tr< the plane of paper through Fig. 49,
"> ^ Vbioil the front Eurfnce of the eje as to ahave off ft
"Wll portion of the posterior surface of tho lens I, but »o
w iu front as not to be carried at all through the stalL
'^ftt u before ; /, the choroidal fissure.

C aBaiou Blong the line t, 1, perpeadicular to the piano of tlio
1"^, to *bew the choroidal fissure/, and the continuity of
'<'* <tnt}- of the optio stalk with that of the primary optic
'•*!^^ Had thia aeetion been takeu a little to either side of
'''' liu j; I, the wall of the optii^ cup would have extended
■f to tbc hus below OS well aa above. Letters ail above.

•*« {the superficial epiblaat of the head would also
P»rfi«*i]j| but there would be nothing Been of either
■e nallt or the fissure. If on the other hand the
Kow were taken in a plane parallel to the plane of
" P*per, at some distance above the level of the
'>lk>B)Die such figure would be gained as that shewn
* Rg. 50 B. Here the fissure f is obvious, and the
■nmumctitJon of the ca\-ity vh of the secondary vesicle

11 the outside of the eye evident ; the section of
•""IW would not go through the superficial epiblaat,

140 THE THIRD DAT. [CHAP.

Lastly, a section, taken perpendicular to the plaoe of
the paper along the line z, i.e. through the fissure
itself, would present the appearances of Fig. 50 C,
where the wall of the vesicle is entirely wanting in the
region of the fissure marked by the position of the
letter /, The external epiblast has been omitted in
the figure.

The fissure such as we have described it exists for
a short time only. Its lips come into contact, and
unite (in the neighbourhood of the lens, directly, but in
the neighbourhood of the stalk, by the intervention of
a structure which we shall describe presently), and thus
the cup-like cavity of the secondary optic vesicle is
furnished with a complete wall all round. The interior
of the cavity is filled by the vitreous humour, a clear
Quid in which are a few scattered cells.

With roferenoo to tiie above description, two points require
to be noticed Firstly it is oitremely doubtful whether the
invagination of the secondar; optio vesicle is to be viewed as an
actual meobanicnl result of the iugrowth of the lens. Secondly
it aeetoB probable thai the choroid li.-wure ia not simply due to a
deficiency in the growth of part of the walls of the eecondaiy
optic cup, but is partly duo to a more complicated inequality of
growth resulting in a doubling up of the primnjy vesicle from
the aide along the line of the fissure, at the some time tbat the
lena is being thrust in in front In Uammalia, the doubling up
■olves the optic stalk, which becomes flattened (whereby its
original cavity ia obliterated) and then folded in on itaelf, so as
1 embrace a new central oavity continuous with the cavity of
the vitreous humour.

During the changes in the optic vesicle just de-
scribed, the surrounding mesoblast takes on the cha-
racters of a distinct investment, whereby the outline of

THE EYE 141

B eyeball U definitely formed. The internal portions
fids investment, Dearest to the retina, become the

iBw'i (i,fl. the chorio-capiUaris, and the lamina
the pigment epithelium, na we have seen, being

•ived from the epiblastic optic cnp), and pigment is
wquently deposited in it, The romaining externa!
itioii of the investment forms the sclerotic.
The complete differentiation of these two coats
the eye does not however take place till a late

Id front of the optic cup the mesoblastic invest-
U glows forwards, between the lens and the super-
ll epiblast, and so gives rise to the substance of
I coniea; the epiblast supplying only the anterior
iUieiium.

We may now proceed to give some further details
Ui reference to the histological differentiation of the
Its, whose general development has been dealt with
the preceding pages.

The histological condition of the eye in its earliest
lg« is very simple. Both the epiblast forming the
llU of the optic vesicle, and the superficial layer
&h ia thickened to become the lens, are composed of
^e columnar cells. The surrounding mesoblast is
ide lip of cells whose protoplasm is more or less
bkched and irregular. These simple elements are
■dually modified into the complicated tissues of the
lit eye, the changes undergone being most marked
the cases of the retina, the optic nerve, and the
■ ^ith its appendages.

tThe optic vesicle. We left the original cavity of
primary optic vesicle as a nearly obhterated space

I
I

142

THE THIRD DAY.

[chap.

between the two walls of the optic cup. By tlie end
of the third day the oblit«ration is complete, and the
two walls are in immediate contact.

The inner or anterior wall is, from the first, thicker
than the outer or posterior ; and over the greater part
of the cup this contrast increases with the growth of
the eye, the anterior wall becoming markedly thicker
and undergoing changes of which we shall have to
speak directly (Fig. 51).

In the front portion however, along, so to speak, the
lip of the Clip, anterior to a line which afterwards be-
comes the ora aerrata, both layers not only cease to
take part in the increased thickening, accompanied by
peculiar histological changes, which the rest of the cup
is undergoing, but also completely coalesce together.
Thus a hind portion or true retina is marked off from a
front portion.

The front portion, accompanied by the choroid
which immediately overlays it, is, behind the lens,
thrown into folds, the ciliary ridges ; while fiirther for-
ward it bends in between the lens and the cornea to
form the iris. The original wide opening of the optic
cup is thus narrowed to a smaller orifice, the pupil;
and the lens, which before lay in the open mouth, is
now inclosed in the cavity of the cup. While in the
hind portion of the cup, or retina proper, no deposit of
black pigment takes place in the layer formed oat of
the inner or anterior wall of the vesicle, in the front
portion we are speaking of, pigment is largely deposited
throughout both layers, so that eventually this portion
seems to become nothing more than a forward pro-
longation of the pigment-epithelium of the choroid.

THE OPTJC VESICLE.

THE Eye of Chick at the Fourth Dat.
epiblaet of tbe nde of the head.
""* fBtina : anterior wall of the optio cup. p. Ch. iiigmeut-
fl'™Bliiun o( the choroid ; jKisterior wall of the optic cup,
* " pUoad at the estreme Up of the optic oup at what
'■Wotoe the mar^ii of the iris.

^o*- The hind wall, the ouclei of whose elongattid cells
*"• •liewn at rd, now forma nearly the whole mass of the lens,
^ "tmt wall being reduced to a. lajer of flatteued cells ei.

"WBoblaat surrounding the optic cup and about to form
* *oreid and Bclerotio. It is seen topassforwardbetvfeen
'l> of tike optic cup and the superficial epibloot.

fill

:1k ^H

DB, ^H

rm ^^H

144

TIIE THIRD DAY.

[CHAl

Filling up a large part of the hollow of the optic oup is seen
a hjoline ax&aa iormiug the hjoloiU mombraue nnd the coftgulum
of the vitreoUB humour. In the ncighboiirhoocl of the lens it
aeema to be ooutinuoua aa aid with the tiaaue a, which in turn
ia oontinuoua with the meaoblast m, and uppeors to be the
rudiment of the capsule of the lena and Buspensor; ligament.

• Thus while the hind moictj of the optic cup be-
comes the retina proper, including the choroid -pigment
in which the rods and cones are imbedded, the front
moiety is converted into the ciliary portion of the
retina, covering the ciliary processes, and into the uvea
of the iris ; the bodies of the ciliary processes and the
Bubatance of the iris, their vessels, muscles, connective
tissue and ramified pigment, being derived from the
mesoblnstic choroid. The margin of the pupil marks
the extreme lip of the optic vesicle, where the outer or
posterior wall turns round to join the inner or anterior.

The ciliaiy muscle and the ligamentum pectinatum
are both derived from the mesoblast between the
cornea and the iris,

The retina. At first, as we have said, the two walls
of the optic cup do not greatly differ in thickness. On
the third day the outer or posterioi becomes mucli
thinner than the inner or anterior, and by the middle
of the fourth day is reduced to a single layer of flat-
tened cells (Fig. 51, p. Gh.). At about the 80th hour
its cells commence to receive a deposit of pigment, and
eventually form the so-called pigmentary epithelium of
the choroid ; from them no part of the true retina (or
no other part of the retina, if the pigment-layer in
question be supposed to belong more truly to the retina
than to the choroid) is derived.

VI.] THE BEHNA. 145

On the fourth day, the inner (anterior) wall of the
optic cup (Fig. 51, R) is perfectly uniform in structure,
being composed of elongated somewhat spindle-shaped
cells, with distinct nuclei On its external (posterior)
surface a distinct cuticular membrane, the membrana
limiUans externa, early appears.

As the wall increases in thickness, its cells multipl}'
rapidly, so that it soon appears to be several cells thick :
each cell being however probably continued throu^li
the whole thickness of the layer. The wall at thin
stage corresponds closely in its structure with the brain.
of which it may properly be looked upon as part. Ac-
cording to the usual view, which is not howr^viir fully
supported by recent observations, the retina \}it(U)UU'H
divided in its subsequent growth mU) (1) an owU^r
part, corresponding morphologic?illy to th^j fijiit.hitlial
lining of the cerebro-spinal canal, comiK/H<;d of what
may be called the visual cells of the trya, i. e. i\it% rullfc
forming the outer granular (nuclear; lay^rr m%i\ th*? ttA^
and cones attached to them ; and ^2; an inrMrf \ntrl\t/it
consisting of the inner granular (wwlfs^r) Uiyf-r, iUt-
inner molecular layer, thf: gangli^/fii/; Ihytrr an/l ih*
layer of nerve-filH'es confaq^/Mltn^ uifffiAuA/f^itM^ u,
the substance of the brain aorJ n\nfis»\ f/fff\

The actoal develr^iCMAt ^f ^i«<t f^ftiri* hk t*fA fi^/t*/^»i^A/
undentood. Aecordm^ tf> tky^, -atnAi ^jif^40*ef4M ^fC/AUWJcf% K)r^
layer of guigiion eeibi saA ^Jc^r tta^^cf u*^M^>*iJtf \^/^ k4n tr*t.
dikerentiirtedy whSe tht ntMxAvft^ ^imU a^>« ri^ v^ ^*a f*aa.
of the letxDa propexv ^knrf ar^ r^^wji^ tcti^t^^u^ ) f^ Vf^ M^ky^i^^^fA*^
limitans extenuL Oft ^ticut jiwuw «*> '4 •a^ ^,^£^^.^ .^/^ KM
stratum rf acrre-fOvw* ji ^iiw. ^^/ *^ ; *ser^/4.*«fc^ 't:^ #>^

THE THIRD DAY. [cHAP.

a are formed a-i prolongtitionB or cutiouloriintiona of the
cells which eventuallj form the outer granular layer. The layer
of uells extornal to the moleauliLr layer is not divided till
comparatively late into the inner aiid outer grauiilar (nuclear)
layerB, and the interpoeed out«r molecular layer.

LQve' has recently written an elaborate paper on this subject
in which ho arrives at very different resuita from KSlliker
and othor observers.

According to him ouly the outer limbs of the rods and
Gonee, which ho holda to be metamorphosed cells, correspond to
the epithelial layer of the braia

The changes described above are confined to that
portion of the retina which lies behind the era serrata.
In front of this both walls of the cup coalesce as ne
have said into a cellular layer in which a deposit of
pigment takes place.

At a very early period a membrane appears on the aide of
the retina adjoining the vitreous humour. This membrane is
the hyaloid membrane. It is formed at a time when there is no
trace of mesoblastio structures ia the cavity of the vitreoua
humour, and must therefore be regarded aa a outicular deposit
of the cells of the optic cup.

The optic nerve. The optic nerves are derived,
as we have said, from the at first hollow stalks of the
optic vesicles. Their cavities gradually become obUte-
rated by a thickening of the walla, the obliteration
proceeding from the retinal end inwards towards the
brain. While the proximal ends of the optic stalks
are still hollow, the rudiments of the optic chiasma
are formed at the roots of the stalka, the fibres of
the one stalk growing over into the attachment of the
other. The decussation of the fibres would appear
' Inhiv^mikr. Aitat. Vol. it.

tl.j THE CHOROID FISSUHfc*

to be complete. The fibres arise in the remainder of
(he nerves aomewhat later. At first the optic nerve
B equally cxtntinuous with both walla of the optic cup ;
must of necessity be the case, since the interval
*bich primarily exists between the two walls is con-
finuous with the cavity of the stalk. When the cavity
nthin tie optic ner\'e vanishes, and the fibres of the
•ptic nerve appear, all connection between the outer
*sll of the optic cup and the optic nerve disappears,
•Bd the optic ner^'e simply perforates the outer wall,
IWMing continuous with the inner one.

Klfl choroid fissnre. During the third day of incu-
"ion there passes in through the choroid slit a vas-
™i« loop, which no doubt supplies the transuded
for the growth of the vitreous humour. Up to
b day this vascular loop is the only structure
through the choroid slit. On this day however
* new structure appears, which remains permanently
■Kiagh life, and is known as the pecteg. It consists
iunellar process of the mesoblast cells round the
9*1 pacing through the choroid slit near the optic
■erre.and enveloping part of the afferent branch of
"6 vascular loop above mentioned. The proximal part
•ilbe free edge of the pecten is somewhat swollen, and
•Wions through this part have a club-shaped form.
On &e sixth day the choroid slit becomes rapidly
"Wed, BO that at the end of the sixth day it is reduced
ere seam. There are however two parts of this
where the edges of the optic cup have not
■**ltsced. The proximal of these adjoins the optic
'■*rve, and permits the passive of the pecten, and at a
*er period of the ojitic nerve ; and the second or distal
10—^

' 148

THE THIRD DAY.

[chap.

one is placud near the ciliary edge of the slit, and is
traversed by the efferent branch of the above-men-
tioned vascular loop. This vessel soon atrophies, and
with it the distal opening in the choroid slit completely
vanishes. In some varieties of domestic Fowl (Lieber-
kiihn) the opening however persists. The seam which
marks the original site of the choroid slit is at first con-
spicuous by the absence of pigment, and at a later
period by the deep colour of its pigment. Finally, a
little after the ninth day, no trace of it is to be
seen.

Up to the eighth day the pecten remains as a simple
lamina; by the tenth or twelftli day it begins to be
folded or rather puckered, and by the seventeenth or
eighteenth day it is richly pigmented, and the pucker-
inga have become nearly as numerous as in the adult,
there being in all seventeen or eighteen. The pecten
is now almost entirely composed of vascular coils, which
are supported by a sparse pigmented connective tissue ;
and in the adult the pecten is still extremely vascular.
The original arterj' which became enveloped at the
Ibrmation of the pecten continues, when the latter Ije-
comes vascular, to supply it with blood. The vein is
practically a fresh development after the atrophy of
the distal portion of the primitive vascular loop of the
vitreous humour.

There are no true retinal blood-vessels.

The permanent opening in the choroid fissure for
the pecten is intimately related to the entrance of the
optic nerve into the eyeball; the fibres of the optic
nerve passing in at the inner border of the pecten,
coursing along its sides to its outer border, and ladi-

n] THE LENS. 14L)

atin^ from it aa from a centie to all parte of the

The lens, Thia when first formed is somewhat
elliptical in section with a small central cavity of a
aoiiar shape, the front and hind walla being of nearly
*<iiial thickness, each consisting of a single layer of
elongate! colunmar cells.

In lie subseqnent growth of the lens, the develop-
oeal of the hind wall is of a precisely opposite cha-
fKier lo tliat of the front wall. The hind wall becomes
much ihicki^r, and tends to obUtcrate the central cavity
by booming conves on its front surface. At the samt;
'"M its cells, still remaining as a_ single layer, becomi'
elongiital and fibre-like. The front wall on the con-
tiMy bwomes thinner and thinner and its cells more
•Jul niure flattened and pavement-like.

Tlipse modes of growth continue until at the end of
•m fourth day, as shewn in Fig. 51, the convex hind
**"' comes inlo absolute contact with the front wall
« and the cavity is thus entirely obliterated. The cells
"• we hind wall have by this time become veritiiblc
•""W, which, when seen in section, appear to be arranged
"Wly parallel to the optic axis, their nuclei nl being
■*n in I row along their middle. TTie front wall, sorae-
*»at tjjickened at either side where it becomes continu-
'"* *ith the hind wall, is now a single layer of flattened
™> Kpamting the hind wall of the lens, or as we may
*■* ay the lens itself, from the front limb of thi-
""•^pBule ; of this it becomes the epithelium.

ine subsequent changes undergone consist chiefly in
"* ooatinued elongation and multiplication of the lena-
1*'^ with the partial disappearance of their nucleL

160 THE TaiHD DAT. [CHAP.

During their multiplication they become arranged
in the manner characteristic of the adult lens.

The lens capsule is probably formed as a cuticnlar
membrane deposited by the epithelial cells of the lens.
But it should be stated that many embryologiste regard
it ae a product of the mesublast.

The vitreous humour. The vitreous humour is a
mesoblastic product, entering the cavity, of the optic
cup by the choroid slit just spoken of. It is nourished by
the vascular ingrowths through the choroid slit. Its
exact nature haa been much disputed. It arises as a
kind of transudation, but frequently however contains
blood-corpuscles and embryonic mesoblastic cells. It
is therefore intermediate in itii character between or-
dinaiy intercellular substance, and the fluids contained
in serous cavitiea.

The integral parts of the eye in front of the lens are
the cornea, the aqueous humour, and the iris. The
development of the latter has already been sufficiently
described in connection with the retina, and there re-
main to be dealt with the cornea, and the cavity con-
taining the aqueous humour.

Tlie cornea. The comea is formed by the coales-
cence of two structures, viz. the epithelium of the
cornea and the cornea proper. The former is directly
derived from the external epibiast, which covers the
eye after the invagination of the lens. The latter is
formed in a somewhat remarkable manner, first clearly
made out by Kcssler.

When the lens is completely separated from the epi-
dermis the central part of its outer wall remains directly

vl] thk corstea. 151

in contact with the epidennis (future corneal ejHthelium).
At its edge there is a small ring-shaped space boonded
by the outer skin, the lens and the edge of the optic cup.
There appears, at about the time when the cavity of
the lens is comjdetely obliterated, a structureless layer
external to the above ring-Eke space and immediately
adjoining the inner face of the epidermis. This layer,
which forms the commencement of the cornea proper,
at first only forms a rii^ at the border of the lens,
thickest at its outer edge, and gradually thinning
away towards the centre. It soon however becomes
broader, and finally forms a continuous stratum of con-
siderable thickness, interposed between the external
skin and the lens. As soon as this stratum has
reached a certain thickness, a layer of flattened ceUs
grows in along its inner side firom the mesoblast sur-
rounding the optic cup (Fig. 62, dm). This layer is
the epithelioid layer of the membrane of Descemet\
After it has become completely established, the meso-
blast around the edge of the cornea becomes divided
into two strata ; an inner one (Fig. 52 cb) destined to
form the mesoblastic tissue of the iris already described,
and an outer one (Fig. 52 cc) adjoining the epidermis.
The outer stratum gives rise to the corneal corpuscles,
which are the only constituents of the cornea not yet
developed. The corneal corpuscles make their way

^ It appears possible that Lieberkuhn may be right in stating
that the epithelium of Desoemet's membrane grows in between the
lens and the epiblast before the formation of the oomea proper, and
that Kessler's account, giyen aboTe, may on this point require oorreo-
tion. From the stmctnre of the eye in some of the lower forms il
seems probable that Descemet's membrane, is continaoos with tK*
choroid.

SBCmOIF THROCSR THE BYK OF 4 FoWL OS THE EIGHTH DAT
', TO BH£W TB£ HUB ASQ CDRNCA rM THE

' F0H«ATI0N, (After Keaaler.)

ep. epiblaatio epithelium of oomeft ; ec corneal cor|iuscleH growing
into the structureless matrix of the cornea ; dm. Descemef s
membrane ; tr. iris ; cb, mesoblaat of the iris [tliia reference
letter points a little too high).

The space between the layers dm. anJ ep. ia filled with the
structureless matrix of the coruea.

through the structureless corneal layer, and divide it
into two strata, one adjoining the epiblaat, and the
other adjoining the inner epithelium. The two strata
become gradually thinner oa the corpuscles invade a
larger and larger portion of their suhstance, and finally
the outermost portion of each alono remains to form
ahove and below the membrana clastica anterior and
posterior (Deacemet's membrane) of the cornea. Tho
corneal corpuscles, which have grown in from the sides,
thus form a layer which becomes continually thicker,
and gives rise to the main substance of the cornea.

Whether the increase in the thickness of the layer is
due to the immigration of fresh corpuscles, or to the
division of those already there, is not clear. After the

THE AQUEOUS HUMOl'R.

Kltulu elements have made their way into the cornea,
ktter becomes continuous at its edge with the meso-
liaM wtdcb fonns the sclerotic.

Ilu ikrivtticin of the original atructurelesa la;er of the coroea
■tin lUHectain. Kesster derives it fnmx the epiUast, but it
pWH luore probable that SillUker' is right in r^ardiog it
"iwi'ed froBi the mesoblast. The grounds for this view are,
I] ue [jKt of its growth inwards from the border of the meao-
nimd the edge of the e^e, (S) the peculiar relations between
!(ad till] ojtueal corpusclea at a hkter period. This view would
iw rtiij further support if a hiyer of musoblast between the
UhI the epiblast were really present as believed by Lieber-
>■ It iniist however be admitted that the objectioos to
imt'i Tictr of its epiblaatic nature are rather a priori than
W on ilefinito observation.

The obeervationa of Keaaler, which have been mainly followed
™ »bove account, are strongly opposed by LieberkUhu and
" obaerverB, and are not entirely acoepted by EQUiker. It
'"nvsr eqwcially oa the development of these parta in Mam-
li* (til be spoktin of in the sequel) that the above authoca
"J iJleir objeclioiuj.

"^ aqueous haiuour. The cavity for the aqueous I

Dour baa itd origin in the ring-shaped space round |
front of the lens, which, as already mentioned, i

tailed by the external skin, the edge ol' the optic cuj^ i
the lens. By the formation of the cornea this

3 is ahut off from the external skin, and on the ,
Pwance of the epithelioid layer

ibnno a continaoua cavity is developed betvreen {

cwnea and the lens. This cavity enlarges and .

^Xaiatt.ZiirEntii>itk.d.Auifind.WirbeUhiere. Lripitg, 187^ I
'•^kOhn, " Beitiikge z. Anat. d. cmbiyonalen Aogcs," Arehto J
".0, Phyi., 1879. EoUiker, EntKick. d. SJetuch^, etc. Cieipzis. J

THE THIRD DAT.

receive its final form upon the full developmenl
iiie.

Summary. We may briefly recapitulate the main
fects in the development of the eye as follows.

The eye commences as a lateral outgrowth of the
fore-brain, in the form of a stalked vesicle.

The stalk, becoming narrowed and subsequently
solid, is converted into the optic nerve.

An involution of the superficial epiblast over the
front of the optic vesicle, in the form first of a pit, then
of a closed sac with thick walls, and lastly, of a solid
rounded mass (the small central cavity being entirely
obliterated by the thickening of the hind wall), gives
rise to the lens. Coincidently with this involution of
the lens, the optic vesicle is doubled up on itself, and
its cavity obliterated ; thus a secondary optic vesicle
or optic cup with a thick anterior and a thin posterior
wall is produced. As a result of the manner in which
the doubling up takes place, or of the mode of growth
afterwards, the cup of the secondary optic vesicle is at
ftrst imperfect along its under surface, where a gap, the
choroidal fissure, exists for some httle time, but subse-
quently closes up.

The mesoblast in which the eye is imbedded gathers
itself together around the optic cup into a distinct in-
vestment, of which the internal layers become the
choroid, the external the sclerotic. An ingrowth of
this investment between the front surface of the lens
and the superficial epiblast furnishes the body of the
cornea, the epiblast itself remaining as the anterior
corneal epithelium.

The mesoblast entering on the under side through

THE LACRYHAL DCCT. 155

boioida] fissure gives rise to the vitreous hamour,
I at a later stage a definite process of this meso-
beoomes the pecten.
Of the walls of the optic cup, the thinner onter
iar) wall becomes, behiml the line of the ora
, the pigment-epithelium of the choroid, vhile
thicker inner [anterior) nail snpplies all the ele-
ts of the retina, inciading the rods and oones vfaicb
ir out from it into the pigments-epithelium,
lb front of the line of the ora serrata, both vails of
optic cup, quite thin and wbollj fused together, give
to the pigment-epillielium of the ciliary processes
iiis, the bodies of both these organs being formed
D the mesoblastic investment.

Aeeeaory Oiyani eotmected iriCA t/ie £ye.
K The most impartant accessor? BtmcturES connected
kUiRe^ are the cj'eluls. Tbejore developed as aiinple folds
btinleguioeDt with a mesoblastic pioloDgation between their
khoinn. The; are thr«e in number, viz. an upper and lower,
• htenl one — the nictitating membrane—GpriD^ng from
'UW w anterior border of the eya. Their inner face ia lined
■ pmloagation of oonjunctlva, which is the modified epifalaat
King the cornea and port of tbo Bclerotic.

The Lacrymal gUnds and Lacrymal dnct.
ue liMymal glands are formed as solid iogrowtha of the

iTil epithelimo. The/ appear ou the eighth day of

»t.
^ lacrrnia) duct begins as a solid ridge of the epidermis,
'"'tiag iuwanls along the line of the so-called lacrTmal groove,
n the eye to the nasal pit

^ Uu! end of the sixth day this ridge beg^los to be separated
" tilt epidermis, remaining however onit^ with it on the
■■ndc of the lower eyelid.

15G TOE THUID DAY. [CHAP.

A&6T it hns become free, it forms a solid cord, the lower end
of wliich unites with the wall of the nasal cavity. The eord
30 fonaod girea rise directly to the whole of the duct proper oud
to the lower branch of the collecting tube. The upper branch of
the collecting tube- is formed as an outgrowth from it. A lumen
begins to be formed in it on the twelfth day of incubation, and first
appears at the nasal end It arises aa a space amongst the cella
of the cord, but in not due to an absorption of the central ctUaK

Organ of hearing. During the second day the ear
first made its appearance on either side of the hind-
brain as an involution of the extemaJ epibtast, thrust
down into the mass of mesoblast rapidly developing;;
^L between the epiblast of the skin and that of the neural

■

H Wiibell

^ v., 187!

aup. auditory

pit; aun.

ganglion

of auditory nerve ;

i>.v. roof

of fourth

entricle ;

a.v.i'. anterior cardinal vein ; a

-7. aorta;

' O.Bora:

. Thranenoawngaag i.

amnioten

Wirbellhiere, r

LacertiliB

a. Atcb.'

«cft, Vol.

v., 1979.

TL]

fint iwBsl ciaft : JL r
oottl fig. Tf", OIL. ji... Ii liieai had iht iami of a
sfaallov jjdt "wtih a indtJT cpem tamnii, ■""'i^'"' ja fonn
to tfau dtevs ^ Kii (aiilff;i=o di^fidi in Fig; aS, au. p.
Before the OKi c€ the third d»j, hs mouth ck»E» up wad
sU BgnB of tbit openisg are oUJunaeid. Ute pdt thns
becomes ooDTated isw a doaod resde, hsed with
epfblast. anA sumNiuded br meaoUasL This resicle is
the otic tttieU, wboat txvay ixftdiy enluges vhile ita
walls become thickened ^. 5ft, 0(7^
Fifi. H.

&n;no5 thbouoh thk Hdcd-Brais or a Chick at the bhd

OF TBK Thibd Day or Ikcubatios.
/r. Fourtli Tantride. The aectioD shews the vory Mxva roof Mid
thicker aidee of the rentricle.

158

THE THIRD DAY.

[chap.

Gt Notochord— (diagrammatic shading).

CV. Antarioroardinal or jugular vein.

CC. Involuted auditory veaiola CO points to tbe end which
wilt form the cochlear caiiaL RL. RecBssua labyrinthL Ay.
hypoblast lining the alimentary conol ky is itself placed in
the cavity of the alimentary canal, in that port of the canal
which will become the throat. The lower (anterior) wall of
the canal is not shewn in the section, but on each side are
seen portions of a pair of visceral arohoa. In each arch
ia seen the section of the aortic arch AOA belonging to the
visceral arch. The vessel thus cut through is running
upwards towards the head, being about to join the dorsal
aorta AO. Had the section been nearer the head, aD<i
carried through the plane at which the aortic arch curves
round the elimentary canal to reach the mesoblaat above it,
AOA and AO would have formed one continuous curved
space. In sections lower down in the back the two aorta,
AO, one on either side, would be found fused into one median

The changes by which this simple otic vesicle is
converted into the complicated system of parts known
as the internal ear, have been much more completely
worked out for Mammals than for Birds. We shidl
therefore reserve a full account of them for a later
portion of this work. Meanwhile a brief statement of
the essential nature of the changes may be useful ; and
will be most conveniently introduced here.

The internal ear consists essentially of an inner
membranous labyrinth lying loosely in and only partially
attached to an outer osseous labyrinth.

The membranous labyrinth (Fig. 55) consists of two
parts ; (1) the vestibule, with which are connected three
pairs of semicircular canals, pag', fr, hor, and a long
narrow hollow process, the aqueductua or receasus vesti-

YL]

Two VUWS OF THB MEKBBANOU8 LaBIBINTH OP COLUMBA

DoKEBTiCA (copied from Haaae).
A, from the eiterior, B, from the interior.
hot'. horizoDtal semicircular canal, Aor. ampulla of ditta,p(^. pos-
terior vertical semicircular canal, pag. ampulla of ditto,
ft', aalerior vertical semicircular canal, ^. ampulla of ditto,
II. utriculuB, ru. receasuB utriculi, v. the connecting tabs
between the ampulla of the anterior vertical s
canal and the utriculus, de. ductus endolymphaticua (rei
vestibuli), *. sacculus hemisphericus, cr. canalis reunicns, lag.
lagena, nir. membrane of Reissner, pfi. Basilar membrane.

bull, and (2) the ductus cocfdearia, which in birds is a
flask-shaped cavity slightly bent on itself, the dilated
end of which is called the lagena. The several parts of
each of these cavities freely communicate, and the two
are joined together by a narrow canal, the canalis re-
uniens, cr.

The osaeous labyrinth has a corresponding form,
and may be similarly divided into parts : into a bony
vestibule, with its bony semicircular canals and recesBU>

160 THE miED DAT. [cHAP.

vestibuli, and into a bony cochlea; but the junction
between the cochlea and the bony vestibule is much
wider than the membranous canalis reuniens.

The cavity of the osseous cochlea ia partially divided
lengthways by the ductus cochlearis into a acala tym-
pimi and a scala vestibuH, which do not however extend
to the lagena.

The auditory nerve, piercing the osseous labyrinth
in various points, is distributed in the walls of the mem-
branous labyrinth.

All these complicated structures are derived from
the simple primary otic vesicle and the surrounding
mesoblast by changes in its form and differentiation of
its walls. All the epiblast of the vesicle goes to form
the epithelium of the membranous labyrinth^ whose
cavity, filled with endolymph, represents the original
cavity which was first open to the surface but subse-
quently covered in. It gradually attains its curiously
twisted form by a series of peculiar processes of unequal
growth in the, at first, simple walls of the vesicle. The
corium of the membranous labyrinth, and all the tissues
of the osseous labyrinth, are developed out of the meso-
blastic investment of the vesicle. The space between
the osseous and membranous labyrinths, including the
scala vestibuh and scala tympani, may be regarded as
essentially a series of lymphatic cavities hollowed out
in the mesoblast.

It will be seen then that the ear, while resembling
the eye in so far as the peculiar structures in which the
sensory nerve in each case terminates are formed of
involuted epiblast, differs from it inasmuch as it aris^
by an independent involution of the superficial epiblast.

n] THE OLFACTORV OHGAK. Iffi

whereas the eye is a constricted portioQ of the general
iuToIatiou which gives rise to the central nervous
lystem.

The origin of the auditory nerve has already
iMcribed. It is shewn in close contact with the wi
of the auditory pit in Fig. 53.

Ol^&n of SmelL The organ of smell makes its ap-
pearance during the third day, as two depressions or
piu, UQ the under surface of the hetid, a little in front
oflheeye(F^. 56. JV).

Fia. 5G.

w^b^^l

"**" OF m Ejibrto Cbick of the Thibd Day

6IDEWATB AB AN QPAQUE OBJECT,

(Chromio acid prepiiratioii.)
^ Cenbral heioispherea. F.B. Veaicle of third i
U.B. Mid-bnuQ. Ch. Cenbellum. H.B. Medulla i
loog&t^

Km*! pit. o(. otic Teeiclo iu the stage of a pit with the opeo-
iiig not yet closed up. op. Optio TeHinle, with I. lens and
oL/ choroidal fisaurc. The superficial epibloat moulds
itself to the (brm of the optic vesicle aud the lens ; hance
the choroidal li&3ur«, though formed entirely underneath the
tnperfidal epiblast, is distinctly visible Irom the outsido.
f. The first viaoeral fold; above It is seen a slight indioatii

the superior maxillary prin-'c^ua.
1^4 F. Second, third and fourth visoeiul folds, with the
dubI clefts between them.
1. 4 B. U

162

THE THIRD DAY.

[chap.

Like the lena and the labyrinth of the ear, they axe
fonned from the external epiblast; unlike them they
are never closed up.

The oliactory nerves arise as outgrowths of the front
end of the cerebral hemispheres, before any trace of a
special division of the brain, fonning an olfactory lobe,
1ms become established. Their peripheral estremities
unite with the walls of the olfactory pits during the
third day. The olfactory lobes arise as outgrowths of
the cerebral hemispheres on the seventh day of incuba-
tion.

Visceral Arches and Visceral Clefts. It must be
Iwme in mind that, especially in the early stages of
development, owing to the very unequal growth of
different parts, the relative position of the various
structures is continually shifting. This is very well
seen in the instance of the heart. At its first appear-
ance, the heart ia lodged immediately beneath the
extreme front of the aiimentaiy canal, so far forwards
as to underlie that portion of the medullary canal which
will form the brain. It is, in fact, at that epoch a part
of the head. From that early position it gradually
recedes farther and farther backward, until, at the end
of the third day, a considerable interval is observed
between it and the actual head. In other words, a
distinct neck has been formed, in which most important
changes take place.

The neck is distinguished from the trunk in which
the heart now hes by the important feature that in it
there is no cleavage of the mesoblast into somatopleure
and splanchnopleure, and consequently no pleuroperito-
neal cavity. In passing from the exterior into the ali-

THE VISCERAL CLEFTS. 163

ffieoUiy canal, the three layers of the blastoderm are

hiccmvely traveised, without any breach of continuity,

1^ BQch as is caused by the cavities of the blood-

srIs. In this neck, so constituted, there appear on

third day certain fissures or clefts, the visceral or

\vU clefts. These are real clefts or slits passing

;llt through the walla of the throat, and are placed in

Ms on either aide across the axis of the alimentary

ul, lying not quite at right angles to that axie and

3ach other, but converging somewhat to the

Idle of the throat in front (Fig. 56). Viewed from

' otitside in either fresh or preserved embryos they

not very distinctly seen to be clefts ; but when they

seen from within, after laying open the throat, their

Miacters as elongated oval slits can easily be recog-

ffled.

Four in number on either side, the most anterior is

be 6rat to be formed, the other three following in auc-

. Their formation takes place from within out-

The hypoblast is pushed outwards as a pouch,

grows tilt it meets the epiblaat, which is then

ten through, while the hypoblast forms a junction

h the epiblaat at the outside of the throat.

No sooner has a cleft been formed than its anterior

3er (t.e. the border nearer the head) becomes raised

) » thick lip or fold, the visceral or branchial fold.

cleft has its own fold on its anterior border, and in

i^tion the posterior border of the fourth or last visceral

Irft is raised into a similar fold. There are thus five

i*»l folds to four visceral clefts (Fig. 56). The last

0 folds however, and especially the last, are not nearly

•hick and prominent as the other three, the second,—

U— 1

164 THE THIRD DAV. [CHAP.

being the broadest and most conspicuous of alt. The
first fold meets, or nearly meets, its fellow in the middle
line in front, but the second falls short of reaching the
middle line, and the third, fourth and fil^h do so in an
increasing degree. Thus in front views of the neck a
triangular apace with its apex directed towards the
head is observed between the ends of the several folds.

Into this space the pleuroperitoneal cavity extends,
the somatopleure separating from the splanchnopleure
along the ends of the folds ; and it is here that the
aorta plunges into the mesoblast of the body.

The visceral clefts and arches to a large extent dis-
appear in the adult, and constitute examples of an inte-
resting class of embryonic organs, whose presence is
only to be explained by the fact that, in the ancestors of
the types in which they are now developed in the
embryo, they performed an important function in the
adult. The visceral arches and clefts are in fact the
homologues of the branchial arches and branchial clefte
of Fishes, which continue to be formed in the embryos
of the higher vertebrate types, although they have
ceased to serve as organs of respiration. The skeletal
structures developed in the visceral arches persist as
the jaw-bones and hyoid bone, but the clefts, with the
exception of the first, become obliterated.

Of the history of the skeletal elements we shall
speak in detail hereafter; meanwhile we may briefly
deal with the general history of these parts.

The first fold on either side, incri.'asing rapidly in
size and prominence, does not, like the others, remain
single, but sends ofif in the course of the third day a
branch or bud-like process from its anterior edge. Thia

TBE VISCERAL ABCHES.

165]

branch, starting from near the doreal begiamng of the'
Kiiii, runs ventral wards and forwards, tonding to meet thft
Wiesponding branch from the fold on the other side, at
I point in the middle line nearer the front of the head
BM lie junction of the main folds. The two branches
lo Dot quite meet, being separated by a median proces,
Itich at the same time grows down from the extrema
JOMof the head, and against which they abut Between
in folds, which are directed somewhat backwards
u the branches which slant forwards, a somewhat
-shaped space is developed which, as the folds
more and more prominent, grows deeper and
In the main folds are developed the man-
ind in the branches the superior nuurUbe: the
toge^haped cavity between them is the cavity of the
ith, and the descending process which helps to
iplet« the Tipper margin of this cavity is called, from
parts which will be formed out of it, the fronto-

Part of the mesoblast of the two succeeding pairs of I
1 folds is transformed into the hyoid bone, which J
I be best considered in connection with the develop- ]
ntof the skull. The two last arches disappear with-
giving rise to any permanent structures.
With the exception of the first the visceral clefts
e obliterated at an early stage of embryonic life ;
. the 6r5t persists, although it loses all trace of its
[inal branchial function and becomes intimately con-
Bed with the organ of hearing, of which in fact it j
s a most essential part, becoming converted into i
I Eustachian tube and tympanic cavity. The outer I
; and the outer part also of the cleft becomej

THE THUU) DAY. [CHAP.

obliterated at an early date, but from the inner part of
the cleft a diverticulum is given off towards the ex-
terior, wMch becomes the tympanic cavity. Tbe inner
part of the cleft itself forms the Eustachian tube, while
its mouth forms the oral aperture of this tube.

The meatus auditorius extemus first appears as a
shallow depression at the region where the closure of
the first visceral cleft takes place. It is in part formed
by the tissue surrounding this depression growing up in
the form of a wall, but the blind eud of the meatus also
becomes actually pushed in towards the tympanic
cavity.

The tympanic membrane is derived from the tissue
which separates the meatus auditorius extemus from
the tympanic cavity. This tissue is obviously consti-
tuted of an hypoblastic epithelium on its inner aspect,
an epiblafitic epithelium on its outer aspect, and a layer
of mesoblast between them, and these three layers give
rise to the three layers of which this membrane is
formed in the adult During the greater part of fcetal
life it is relatively very thick, and presents a structure
bearing but little resemblance to that in the adult

The tympanic cavity is bounded on its inner aspect
by the osseous investment of the internal ear, but at
two points, known as the fenestra ovalis and fenestja
rotunda, the bone is deficient and its place is taken by
a membrane.

These two fenestra appear early, and are probably
formed by the nonchrondrification of a small area of
the embryonic cartilage. The upper of the two, or
fenestra ovalis, contains the base of a bono, known as
the columella. The main part of the columella ih

VI.]

TEE AOBTIC AliCHES.

167

formed of s. stalk which is held by Parker to be derived
from part of the skeleton of the visceral arches, while
the base, forming the stapes, appears to be an inde-
pendent formation.

The stalk of the columella extends to the tympanic
membrane; its outer end becoming imbedded in this
membrane, and serving to tranamit the vibrations of
the membrane to the fiuid in the internal ear.

Vaacnlar system. By the end of the second day
three pairs of aortic arches had been established in
connection with the heart. When the visceral folds
and clefts are formed, a definite an'angement between
them and the aortic arches is always observed, The
first visceral cleft runs between the first and second
aortic arches. Consequently the first aortic arch runs
in the first visceral fold, and the second in the second.
In the same way, the second visceral cleft lie.? between
the second and third aortic arches, thL' third aortic arch
running in the third visceral fold. Each aortic arch runs
in the thickened mesoblast of the corresponding fold.

Arrived at the dorsal surface of the alimentary canal,
these arches unite at acute angles to form a common
trunk, the dorsal aorta (Fig. 57, -^i-O), which runs along
the back immediately under the notochord. The lengtli
of this common single trunk is not great, as it soon
divides into two main branches, each of which, afler
giving off the large vitelline artery, Of.A., pursues its
course with diminished calibre to the tail, where it in
finally lost in the capillaries of that part.

The heart is now completely doubled up on itaelf.
Its mode of curvature is apparently somewhat compli-
cated. Starting from the point of junction of the vitel-

THE THIRD DAT.

f THE Abterial Circulation on i
Third Day.
The firel three peira of aortic aroliea. A. The vessel
formed by tho junction of the three pairs of arches. A.O.
Dorsal aorta formed bj the juaction of the two branches A
and A ; it qtiiokly divides again into two branches which
B down one on each side of the notochord. Prom each of
a given off a large branch O/.A., the vitelline artery.
E.CA, I.CA, eitemal and internal carotid arteries.

line veioB (Fig. 37, St), there is first a slight curvature
towards the left ; this is followed by a turn to the right,
and then the heart is completely bent on itaclf, so that
afterwards it pursues a course directed from behind
quit* straight forwards (except perhaps for a. little incli-
nation to the left) to the point where the aortic arches
branch off. In this way, as shewn in section in Fig. 59, A,
the end of the bulbus arteriosus (») comca to lie just
underneath (or in front of according to the position of

the embiyo) that part which has already been marked
oS by the tatera] bulgings as the auricular portion (au).

That part of the heart whiuh is turned to the right,
including the point of doubling up, is the ventricular
portion, and is even at this stage separated from the
auricular portion by a alight neck. This external con-
striction corresponds to au internal narrowing of the
lumen of the heart, and marks the position of the future
caTtalis auricularis.

The ventricular portion is, on the other hand, like-
wise separated by a fainter constriction from the ante-
rior continuation of the heart which forms the bulbus
arteriosus. The projecting part where the doubling
takes place is at this stage still quite round; we shall
see that later on it becomes pointed and forms the apex
of the heart.

The whole venous portion of the heart {if we may so
leak of it, though of course at this stage blood of the
same quality passes right along the whole cardiac can&I)
lies in a plane which is more dorsal than the arterial por-
tion. The point at which the venous roots of the heart,
I.e. the two vitelhne trunks, unite into a single canal, 18
on this day carried farther and farther away from the
heart itself. By the end of the day there is a consider-
able distance between the auricular portion of the actual
heart and the point where the venous roots separate,
each to pursue its course along the splanchnopleure-fold
of its own aide. This distance is traversed by a single
venous trunk, of which the portion close to the auricles
is called the sinus venosus, and the more distant the
ducttis venostis. We shall give to the whole trunk the
name used by the older observers, the meaius venosus.

170

THE THIBD DAY.

[chap

Small arteries to various parts of the body are now
being giveu off by the aorta and its branches. The
capillaries in which these end are jjathered into veins
which unite to form two main trunks on either aide, the
cardinal veins, anterior and posterior (Fig. 36, Fig. 58
Fio. 63.

DiACBAa OF THE VEiforB Cikculaiion ok thk

Third Day.

ff. Heart. J. Jugular or anterior cardinal vein. C. Inferior

or poaterior cardinal veiu. Of. Vitelline vein. dc. Ductus

Cuvieri.
J and C), which nm parallel to the long axis of the body-
in the upper part of the mesoblast, a little external to
the mesoblastic somites. These veins, which do not
attain to any great importance till well ou in the third
day, unite opposite to the heart, on each side, into a
short common tmnk at right angles to themselves.
The two short truidcs thu.s formed, which bear the
name of duclun Cuvieri (Fig. 36, Fig. 5S, dc), running
ventralwards and then transversely straight inwards
towards the middle line fall into the sinus venosus.

The two ductus Cuvieri pass from the heart to the
body walls in a special horizontal mesenterj', whose for-
mation and function we shall return to in speaking of
the formation of the pericardial cavity. The position of
one uf them is shewn in section in Fig. 59 B, dc.

Trans VER3E Sectiohs tbbopoh a Chick Embkvo with

TWENTT-OKB MeBOBLAHTIC SoMITEa TO SHEW THE FOR-
MATION OP THE Pkricabdjal Cavity, A- being the

ANTERIOR SBCTION.
pp. bodj cavity ; pa. pericardial cavity ; al. alimentAry canty ■
au. auricle ; v. ventride ; m, eiaus tqdobus ; do. ductus
Cuvieri ; no. aorta ; mp, iiiuscle-plttte ; mc. medullary cord.

The alimentary canal. As we stated above, thi.'

, folding in of the splanchnopleure to form the alimentary

canal ia proceeding with great rapidity, the tail-fold an

well as the head-fokl contributing largely to this result

The formation of the tail-fold is very similar to that
of the head-fold. The tail is a solid, somewhat curved,
blunt cone of mesoblast, immediately coated with the

THE THIRD DAY. [CHAP-

Buperficial epiblast except at the upper surface {corre-
sponding to the back of the embryo), where lies the
pointed termination of the neural tube.

So rapid is the closure of the aplanchnopleure both
in &ont and behind, that t'wo of the three parts into
which the digestive tract may be divided, are brought,
on this day, to the condition of complete tubes.

The first division, including the region from the
mouth to the duodenum, is completely folded in by the
end of the day; ao likewise ia the third division com-
jnrising the large intestine and the cloaca. The middle
division, corresponding to the future small intestine,
8till remains quite open to the yolk-sac below.

The attachment of the newly formed alimentary
canal to the body above is at first very broad, and only
a thin stratum of mesoblast separates the hypoblast of
the canal from the notochord and mesohlastic somites;
even that maybe absent under the notochord. During the
tiiird day, however, along such portions of the canal as
have become regularly enclosed, i.e. the hinder division
and the posterior moiety of the anterior division, the
meaoblastic attachment becomes narrower and (in a ver-
tical direction) longer, the canal appearing to be drawn
more ventralwards (or according to the position of the
embryo forwards), away from the vertebral column.

In what may be regarded as the pleural division of
the general pleuroperitoneal space, along that part of
the alimentary canal which will form the oesophagus,
this withdrawal is very slight (Fig. 59), but it is very
marked in the peritoneal space. Here such parts of the
digestive canal as are formed como to be suspended from
the body above by a narrow flattened band of mesoblas-

tk tissue which reaches from the neighbooiitood of t
toucbonl, and becomes coDtinuoua with the n
lie coating which wraps roand the bTpoblast of i
WiaL This flattened bond i^ the t ■

THE (ESOPBAOCSl

^nciug in Fig. 65, and much move adraaoed in
In^ 68, H. It is covered on either mde by a Ujer of
bt cvlls forming the epithelioid lining of the peiitoDeal
DmbraQe, whilt! its interior ia compoaed of iii£ffefeiit
km.

Tlie front diriaoD of the digestive tnct coDBsta of
bee puts. The most anterior pan. the SMpkaj^ttf.
ending blindly in front reaches back as fitf ai the
ml uf the hind end of the heart ; and is dtTided into
re^oas, vis. aa anterior region, characierized by tbe
KBce of the visceral clefta, whose deveioiment has
heady been dealt with, and a posteiior teffxn wilhoat
hdefta.

Its transverse section, whii^ up to the end of the
•md da; was somewhat ci«scent-shaped, with tbe
evezity downwarxls, becomes on this day more nearif
teolar. Close to its hinder limit, the lai^s (Fig. 60,
), of whose formation we shall apeak directly, take
it origin.

Tbe portion of the digestive canal which sooceieds
be cesophagns, becomes towards the ckne of the thiid
|9 Bomewbat dilated (Fig. 60, Bi); the region of the
modi is thus indicated.

The hinder or pyloric end of the stomach is separated
a very sraaU interval from the point where the eom-
te cksing in of tbe alimentar7 canal ceases, and where
I spiaochnopleuTfrlolds spread out over the J™*
rt tract is nevertheless clearly markeJ <»t as

THE THIRD DAY. [CBAP. J

DiAOBAU or A PORTION OF THE DiOEBTIVK TrACT OF A

CmoK UPON THE Fourth Day.
(Copied from OOtte.)

The black iooer line represents the hj3>ubla£t, the outer shading
the meaoblast. l;/. lung-divorticulum with expanded termi-
nation, forming the primary lung-vesicle. Si. stomach.
I. two hepatic diverticula with their termiuntions united by
cords of hypoblast cells, p. diverticuluni of the paucieaa
with the vesicular diverticula coming from IL

the duodenum bj the fact that from it, as W6 shall
presently point out, the rudimonts of the ducts of the
liver aud pancrcaa are beginning to bo formed.

The posterior division of the digestive tract, cor-
responding to the great intestine and cloaca, ia &om
its very first formation nearly circular in section and
of a lai^er bore than the cesophagus.

During part of the third day thu hinder end of this
section of the gut is in communication with the neural
tube by the neurenteric canal already spoken of {Fig.
61, ne). The coimnuiiication between the two tubes

DlAGEAMMATIC LONQITCBISAL SECTION THBOUOH THH POB-
TEBtOK EKD OF AS EmbBYO BIRO, &T TB3 TIUE OF TSS
F0BM4TIOM ON THE AiJjANTOIB.

ep. epiblaat; Sp.c spmal ciinoJ ; ck. notochord ; n.e. Deureoteric
canal ; hy. bypoblaat ; p.a.g. postanal gut ; pr. remoina of
primitive stretik folded in on tlte ventral side ; □/. allantoia ;
me, mesoblaat ; an. point where anus will be farmed ; p,c
perivisceral covity ; am. amnion ; to. aomatopleure ; gp.
Bpknc hnopleiire.

does not last long, but even after its rapture thei'e re-
muns a portion of thii canal continuous with the gut ;
this, however, constitutes a purely embryonic and tran-
sient section of the alimentary canal, and is known
as the postanal gut. Immediately in front of it ia a
deep infolding of the cpiblaat, which nearly meets the
hypoblaat (Fig. 61, an) and forms the j-udiment of the
anus and of the outer section of the cloaca into which
the bursa Fabricii opens in the adult. It is known to
embryologists as the proctodcBum, but does not open
into the alimentary tract till considerably later. The

176

THE THIKD DAY.

[CHAI.

section of the alimentary tract immediately in tnnlV
the postanal gut is somewhat enlarged, and becomesAl
inner section of the adult cloaca receiving the \
and genital ducts. The allantois, to whose develop
ment we shall return directly, opens into it ventraUj.

It is to be noted that the two sections of the clouk
of adult birds have a different origin. The inner fiecOMl
being part of the primitive alimentary tract and lined bj
hypoblast; the outer being part of an involution of the
outer skin and lined by epihlast.

The lungs are in their origin essentially buds o
processes from the primitive cesophagus.

At a point immediately behind the region of tha
visceral clefts the cavity of the alimentaij canal \»
comes compressed laterally, and at the same lime a
Btricted in the middle so that its transverse section (Hg
62, 1) is somewhat hourglass-shaped, and shews an uff
or dorsal chamber d, joining on to a lower or ventn
chamber I by a short narrow neck.

The hinder end of the lower tube enlarges (Rg, fi|
2), and then becomes partially divided into two Joba
(Fig. 62, 3). All these parts at first freely communicJiW,
but the two lobes behind, partly by their own growth,
and partly by a process of constriction, soon beco
isolated posteriorly (Fig. 60, Ig]; while in frost tl
open into the lower chamber of the oesophagus.

By a continuation forwards of the process of 0(
striction the lower chamber of the oesophagus, <
with it the two lobes above mentioned, becomes g!
ally transformed into an independent tube, opening
front by a narrow sUt-like aperture into the o
The single tube in front is the ludiment of the t

VI.]

THE LUNGS.

177

and larynx, while the two diverticula behind (Fig. 60,
Ig) become the bronchial tubes and lungs.

While the above changes are taking place in the
hypoblastic walls of the alimentary tract, the splanchnic

Fia. 62.

FOUB DiAQBAMS ILLUSTRATINQ THE FORMATION OF THE

LuNQS. (After Qdtte.)

a. mesoblast ; h, hypoblast ; d. cavity of digestive oanal ; I,
cavity of the pulmonary diverticulum.

In (1) the digestive canal has commenced to be constricted
into a dorsal and ventral canal ; the former the true alimentary
canal, the latter the pulmonary tube ; the two tubes communi-
cate with each other in the centre.

In (2) the ventral (pulmonary) tube has become expanded.

In (3) the expanded portion of the tube has become con-
stricted into two tubes, still communicating with each other and
with the digestive cauaL

In (4) these are completely separated from each other and
from the digestive canal, and the mesoblast has also b^gun to
exhibit externally changes corresponding to the internal changes
which have been going on.

F.&B. 12

■ 178

THE THIRD DAT.

[chap.

mesoblast aurroundiiig these structures becomes very
much thickened; but otherwise bears no marts of the
internal changes which are going on, so that the above
formation of the lungs and traehca cannot be seen from
the surface. As the paired diverticula of the lungs grow
backwards, the mesoblast around them takes however
the form of two lobes, into which they gradually bote
their way.

The further development of the lungs is, at fiiBt,
essentially similar to that of a racemose gland. From
each primitive diverticulum numerous branches are
given off. These branches, which are mainly confined
to the dorsal and lateral parts, penetrate into the Bur-
roundiog mesoblast and continue to give rise to second-
ary and tertiary branches. At right angles to tbe
finest of these the arborescent branches so charac-
teristic of the avian lung are given off. In the meso-
blast around them numerous capillaries make their
appearance.

The air sacs, which form such important adjuncts
of the avian lungs, are the dilated extremities of
the primary pulmonary diverticula and of their maia
branches.

The whole pulmonary structure is therefore the
result of the growth by budding of a system of branched
hypoblastic tubes in the midst of a mass of mesoblaatic
tissue, the hypoblastic elements giving rise to the epi-
thelium' of the tubes and the mesoblast providing tiie
elastic, muscular, cartilaginous, connective and other
tissues of the tracheal and bronchial walls.

Tlie liver is the first formed chylopoietic appendage
of the digestive canal, and arises between the 55th and- ,

THE LIVER.

60tli hour as a couple of diverticula one from either
aide of the duodenum immediately behind the stomach
(Fig. 60, i). These diverticula are of course lined by
hypoblast. The right one ia, in all cases, from the first
longer, but of smaller diameter than the left. Situated
little behind the heart, they embrace between them
"tiie two vitelline veins forming the roots of the meatus

The diverticula soon give r^e to numerous hollow
branches or procesaea, which extend into the surround-
ing mesoblast.

Towards the end of the third day there may further
Iw observed in the greatly thickened mesohlastic invest-
ment of either diverticulum a number of cylindrical
solid cords of hypoblast which are apparently out-
growths from the hypoblast of the branches of the di-
verticula. These cylinders rapidly increase in number,
apparently by a process of sprouting, and their some-
what swollen peripheral extremities come into contact
and unite. And thus, about the ninetieth hour, a sort
of network of solid thick strings of hypoblastic cells is
formed, the mesoblast in the raeahes of the network
becoming at the same time largely converted into
blood-vessels. Each diverticulum becomes in this way
surrounded by a thick mass composed partly of solid
cylinders, and to a less extent of hollow processes, con-
tinuous with the cylinders on the one hand, and the
nuun diverticulum on the other, all knit together with
commencing blood-vessels and unchanged mesoblastic
tissue. Between the two masses runs the now fused
roots of the meatus venosus with which the blood-
vessels in each mass are connected,

180 THE THIRD DAY. [CHAP,

Early on the fourth day each mass sends out ventml
to the meatus venosus a solid projection of hypoblae-
tic cylinders towards its fellow, that from the left side
being much the longest. The two projections unite
and form a long solid wedge, which passes obliquely
down from the right (or from the embryo lying on its
left aide, the upper) mass to the left (or lower) one. In
this new wedge may be seen the same arrangement of a
network of hypoblaatic cylinders filled in with vaacular
meaoblast as in the rest of the liver. The two original
diverticula with their investing masses represent respec-
tively the right and left lobes of the liver, and the wedge-
like bridge connecting them is the middle lobe.

During the fourth and fifth days the growth of the
solid, lobed liver thus formed is very considerable; the
hypoblaatic cylinders multiply rapidly, and the network
formed by them becomes very close, the meshes contain-
ing little more than blood-vessels. The hollow processes
of the diverticula also ramify widely, each branch being
composed of a lining of hypoblast enveloped in a coating
of spindle-shaped mesoblastic cells. The blood-vessels
are in direct connecrion with the meatus venosus — ^have
become, in fact, branches of it. It may soon be observed,
that in those vessels which are connected with the pos-
terior part of the liver (Fig. 74), the stream of blood is
directed from the meatus venosus into the network of
the liver. In those connected with the anterior part the
reverse is the case ; heie the blood flows from the liver
into the meatus venosus. The thick network of solid
cylinders represents the hepatic parenchyma of the adult
liver, while the hollow processes of the diverticula are
the rudiments of the biliary ducts; and we may suppose

eich jolid cylinder to represent a duct with its lumen
ilmott, bat perhaps not quite, completely obliterated.

Doling the fifth day, a special sac or pouch Is deve-
loped from the right primary diverticulum. This pouch,
oniaadag of an inner coat of hypoblast, and an outer of
Btwiblast, is the rudiment of the gall-bladder.

The Pancreas arises nearly at the same time as the
het ia the form of an almost solid outgrowtli from the
ooiBil ade of the intestine nearly opposite but slightly
bekind the hepatic outgrowths (Fig. 60, p). Its blind
comes somewhat enlarged and from it numerous
firaticula grow out into the passive splanchnic meso-

As the ductules grow longer and become branched,
'isciilar processes grow in between them, and the whole
"ini a compact glandular body in the mesentery on
w doraal side of the alimentary tract The primitive
"rtpowth elongates and assumes the character of a duct
On the taxtii day a new similar outgrowth from
Kihodenum takes place between the primary diver-
Bulnm and the stomach. This, which ultimately
lesees with its predecessor, gives rise to the second
*, and forms a considerable part of the adult pan-
is. A third duct ia formed at a much later period.

The ThTTOid body. The thjToiU bod; arises at the end of
■ iNood or beg^nniug of the third daj as an uutgrowth from
l> hfpoUaBt of the ventral wall of the throat opposite the
Bt «f origiii of the anterior aortic arch. It has at first Uie
BD of a groove extending forvarda up to the future tociuth, and
' Hi boat part extending ventrally to the epibiaat. It hao not
n made out whether the wLole groove becomea converted into
> (BROaoeot ihjToi'l By the foiutL day it beicomes a
U HUM of cells, and bjr the fifth ceases bo be connected

182 THE THIRD DAY. [CHlP.

with the epithelium of the throat, becoming at the same tiM
bilobed. By the seventh day it has travelled aomewlal biA-
wards, and the two lobes have completely separated Cnm mA
other. By the ninth day the whole is JDvested by a capnli "C
Donnective tissue, which sends in septa dividing it into a amnltf
of lobes or solid massea of cells, and by the HiiteenthdayitiM
lobes are composed of a number of foUicles, each with a 'o
braoa propria,' and separated from each other b; aepts of
nective tissue, much as io the aditlt'.

The spleen. Although the spleen c&nuot be re^
amoDgHt the glands of the alimentary tract its development uj
conveniently be dealt with here. It is formed shortly after tbi
first appearance of the pancreas, aa a thickeoing of the n*
sentery of the stomach (mesogastrium) and is therefore e
a mesoblastio structure. The mass of meaobls«t which (Mi
the spleen becomes early separated by a groove on the one
&om the pancreas and on the other from the meeentery. S
of ita cells become elongated, and send out processM lH
uniting with like processes from other cells, form the tnbtcnll
system. From the remainder of the tissue are derived the o^
of the spleen pulp, which frequently contain more than «
nucleus. Especial accumulations of these toko place at
period to form the so-called Malpighian corpuscles of the i

The Allantois. We have already had occaaon U
point out that the allantois is essentially a diverticulum
of the alimentary tract into which it opens immediatalj
in front of the anus. Its walls are formed of vs
splanchnic mesoblast, n'ithiu which is a lining of h|fpA
blast It becomes a eonspicuous object on the tiuS
day of incubation, but its first development takes
at an earlier period, and is intimately couneuted
the formation of the posterior section of the gut

At the time of the folding in of the binder end 0
; Entwiehtlung der Schilddriitf. it

e splitting of the niesoblast into somatopleuro
inopleure has extended up to the border of
K hinder division of the primitive streak . The ventral
II of what we have abeady termed the postanal
don of the alimentary tract is formed by the primi-
Ine streak. Immediately in front of this is the involu-
fiun which forms the proctodaeum ; while the wall of
&£ hiadgut in front of the proetodEeum owes ite origin
tosiblding in of the splanchnopletire.

The allantois first appears as a narrow diverticulum
fcnned by a special fold of the splanchuopleure just in
ftonl of the proctodaeum. This protuberance arises, how-
Wybefore the aplanchnopleure has begun to be tucked
1 n u to form the ventral wall of the hindgut ; and it
*beti fonna a diverticulum (Fig. 63 A, All) the open
(nd of which is directed forward, while its blind end
fOtnts somewhat dorsalwards and towards the peritoneal
^We behind the embryo.

As the hindgut becomes folded in the allantois shifts
ih position, and forms (Figs. 63 B and 61) a rather wide
e lying immediately ventral to the hind end of the
■jgestaTe canal, with which it communicates freely by a
Hill conraderable opening ; its blind end projects into
tte pleuroperitoneal cavity below.

1 later the allantois grows forward, and becomes
■ bige spherical vesicle, still however remaining con-
tacted with the cloaca by a narrow canal which forms
it> neck or stalk (Fig. 9 G, at). From the Srst the
I liea in the pleuroperitoneal cavity. In this
y it grows forwards till it reaches the front limit of
K hindgut, where the splanchuopleure turns back to
> tlie jolk-eac. It doea not during the tiiird

THE THIBD DAY.
Fia. 63.

[chap.

Two LoNoiTUDiNAL Sections of thb Tail-end op an Em-
bryo CfllOK TO BHKW THE OltlGIN OF IHl! ALIANTOIB.
A AT THB BKQINNINQ OP THE ThIRD DaY ; B AT THB

MiDDLB OF THE Third Dat. (After Dolirjuin.)

t. the tail ; m. the meeoblast ; ^. the epiblnst ; nf'. the neural
canal ; Dd. the dorsal wall of the hindgut ; SO. somato-
pletire ; Spl. splauchiiopleure ; u. the mesoblaat of the
splanchnopleure earrying the vessels of the yolk-sac j fp.
pleuroperitoneul cavit; ; Df. the epithelium tiQing the
pleuroperitoDeal cavitj ; All. the uommencing allantoia ;
10. projectioii formed by auterior and [wsterior divisioiia of
the primitiTe streak \ y. hypubket which will form the
ventral wall of the hindgut ; v. anal invagitiatiou (procto-
diemu) ; 6*. cloaca.

day project beyond this point; but on the fourth day
begins to pass out beyond the body of the cliick, along
the as yet wide spat;e between the splanchnic and soma^
tic stalks of the embryo, on its way to the space between
the external and internal folds of the amnion, which it
will be remembered, is directly continuous with the
pleuroperitoneal cavity (Fig. 9 K). In this space it

THE MESOBLASnC SOMITES.

185

eventually spreads out over tlie whole body of the
chick. On the first half of the fourth day the vesicle is
Btill very small, and its growth is not very rapid. Ita
mesoblast wall still remains very thick. In the latter
half of the day its growth becomes very rapid, and it
forms a very conspicuous object in a chick of that date
(Fig. 67, Al). At the same time its blood-vessela be-
come important It receives its supply of blood tram
two branches of the aorta kuown as the allantoic arte-
ries, and the blood is brought back from it by two allan-
toic veins which run along in the body walls, and after
uniting into a single trunk fall into the vitelline vein
close behind the liver.

Mesoblast of tlie tnrnk. Ckiincidently with the
appearance of these several rudiments of important
organs in the more or less modified splanchnopleure-
folds, the solid trunk of the embryo is undergoing
marked changes.

When we compare a transverse section taken through
say the middle of the trunk at the end of the third day
(Fig. 65), with a similar one of the second day (Fig. 34),
or even the commencement of the third day (Fig, 64),
we are struck with the great increase of depth (from
dorsal to ventral surface) in proportion to breadth. Thia
is partly due to the slope of the aide walls of the body
having become much steeper, as a direct result of the
rapidly progressing folding off of the embryo from the
yolk-sac. But it is also brought about by the great
changes both of shape and structure which are taking
place in the mesoblastic somites, as well as by the
development of a mass of tissue between the notochord
and the hypoblast of the alimentary canal.

186 THE THIRD DAY. [CHAP,

It will be remembered that the horizontal splitting

of the meaoblast into somatic atid splancbnic layers

extends at first to the dorsal summit of the vertebral

plates, but after the formation of the somites the split

Fia. ei.

"PS

sp.t

Trakstkhsb Section thboddh the Tbcke op a Duck
Embbto with abodt twkntt-fouk MBsoBLAario So-

UITTS.

on. Bmniou ; jo. somatopleure ; ^. sploDcUuopk'ure ; vd. Wolf-
fian duct; It oegmental tube; ca.u. cardinal vein; mt.
muscle 'plate ; tp.ff. spina! ganglion ; tp.c, spinal oord ) cA,
Dotochoni ; ao. aorta ; hff. hypoblast.

between the somatic and splanchnic layers becomes to
a loi^ extent obliterated, though in the anterior somites

vl] the muscle-plates. 187

it appears in part to persist. The somites on the second
day, as seen in a transverse section (Fig. 34, P.v), are
somewhat quadrilateral in form but broader than they
are deep.

Each at that time consists of a somewhat thick
cortex of radiating rather graniilar columnar cells,
enclosing a small kernel of spherical cells. They are
not, as may be seen in the above figure, completely
separated &om the ventral (or rather at this period
lateral) parts of the mesoblastic plate, and the dorsal
and outer layer of the cortex of the somites is continuous
with the somatic layer of mesoblast, the remainder of
the cortex, with the central kernel, being continuous
with the splanchnic layer. Towards the end of the
second and beginning of the third day the dorsal and
outer layer of the cortex, together probably with some
of the central cells of the kernel, becomes separated
off as a special plate. From this plate, which is
shewn in the act of being formed in Fig. G^?, ms, the
greater part of the voluntary muscular system of the
trunk is developed. When once formed the muscle-
plates have in surface views a somewhat oblong form,
and consist of two layers, an irmer and an outer, which
enclose between them an almost obliterated central
cavity (Fig. 65, mp). No sooner is the muscle-plate
formed than the middle portion of the inner layer be-
comes converted into longitudinal muscles. The central
space in the muscle-plates is clearly a remnant of the
vertebral portion of the body cavity, which, though it
wholly or partially disappears in a previous stage, re-
appears again on the formation of the muscle-plate.

It is especially interesting to note that t\i^ ^tsX»

THK DoBSAL Region or an Embryo Chick

AT THE END OF THE ThIRD Da7.

Jm. amniotL m.^. muaole-plate. (' F. cardinal vein. Ao. donal
aorta. The section passea through the point where the
dorsal aorta is just cammenciiig to divide into two branohea.
Ck notoohord. IT. d. Wolffian duct. W. b. conmienciBg
differentiation of the mosoblost cells to form the WoUfifm
body. ep. epiblast. SO. aomstopleiire. Sp. splaiictmo-
pleure. Ay. hypoblast. The section passea through the
point where the digestive canal comiuunicates with the jolk-
sac, and ia consequently still open below.

This section should be compared with the section throu^
the dorsal region of nn embryo at the commeDoament of tha third

yl] the intsbmediate cell-mass. 189

day (Fig. 64). The chief differences between them arise from
the great increase in the space (now filled with mesoblast-cells)
between the notochord and the hypoblast. In addition to this
we have in the later section the completely formed amnion, the
separation of the muscle-plate from the mesoblastic somites, the
formation of the Wolffian body, etc.

The mesoblast including the Wolffian body and the muscle-
plate (m./7.) is represented in a purely diagrammatic manner.
The amnion, of which only the inner limb or true amnion is
represented in the figure, is seen to be composed of epiblast and a
layer of mesoblast ; though in contact with the body above the
top of the medullary canal, it does not in any way coalesce with
it, as might be concluded from the figure.

formed muscles in embryo birds have an arrangement
like that which is permanent in fishes; being longi-
tudinal in direction, and divided into segments.

The remainder of the somites, after the formation of
the muscle-plates, is of very considerable bulk ; the cells
of the cortex belonging to them lose their distinctive
characters, and their major part becomes converted, in a
manner which will be more particularly described in a
future chapter, into the bodies of the permanent ver-
tebrae.

We may merely add here that each of these bodies
sends a process inwards ventral to the medullary cord,
and that the processes from each pair of these bodies
envelope between them the notochord.

The intermediate cell-mass and the Wolffian body.

In a transverse section of a 45 hours' embryo a consider-
able mass of cells may be seen collected between the meso-
blastic somites and the point where the divergence into
somatopleure and splanchnopleure begins (Fig. 34, just
below W^. This mass of cells, which we ha.^^ ^^»&3

/
^t

190 THE THIRD DAT. [CEU.

spoken of as the intermediate cell-masa, is at firet inJ*
tinguishable from the cells lining the inner end of tliE
body cavity ; but on the third day, a special peritoMil
lining of epithelioid cells is developed which ia moieff'
less sharply marked o£F from the adjoining part of tk
intermediate cell-mass. This latter now also pant
without any very sharp line of demarcation into tlB
mesoblastic somite itself; and as the folding in of IIk
side wall progresses, the mass of cells in this poatin
increases in size and grows in between the notochaii'
and the hypoblast, but does not accumulate to a Bof
cient extent to separate them widely until the end
the third or beginning of the fourth day.

The fnHiou between the iatermediate cell-mass and (lie in
portions of the somites becomes so complete on the thiid ill
that it is almost impossible to saj which of the cells ia tte
neighbourhood of the notochord are derived from the
and which form the intermediate ceU-masa. It seemB llsxil
certain however that the cells which form the immediate
meat of the notoohonl really belong to the somites.

The intermediate cell-mass ia of special importu'*

to the embryologist, in that the excretory and g
systems die developed from it.

We have already described (p. 106) the developmS
of the Wolffian duct, and we have now to deal with (l
Wolffian body which is, as the reader has no ism
gathered, the embryonic excretory organ.

The structure of the fully developed Wolffian bod
is fundamentally similar to that of the permanent k
neys, and consists essentially of convoluted tubule
commenciug in Malpighian bodies with vascular glomo
niH, and opening into the duct.

VI.] THE WOLFFIAN BODY. ^ 191

The tubules of the Wolffian body are developed
independently of the Wolffian duct, and are derived
firom the intermediate cell-mass, shewn in Fig. 34,
between the upper end of the body-cavity and the meso-
blastic somite. In the chick the mode of development
of this mass into the segmental tubules is different in
the regions in front of and behind about the sixteenth
segment. In front of about the sixteenth segment
special parts of the intermediate cell-mass remain
attached to the peritoneal epithelium, on this layer
becoming differentiated ; there being several such parts
to each segment The parts of the intermediate cell-
mass attached to the peritoneal epithelium become
converted into S-shaped cords (Fig. 64 st) which soon
unite with the Wolffian duct (wd), and constitute the
primitive Wolffian tubules. Into the commencement
of each of these cords the lumen of the body-cavity is
for a short distance prolonged, so that this part con-
stitutes a rudimentary peritoneal funnel leading from
the body-cavity into the lumen of the Wolffian tubule.

In the foremost Wolffian tubules, which never reach
a very complete development, the peritoneal funnels
widen considerably. The section of the tube adjoining
the wide peritoneal funnel becomes partially invaginated
by the formation of a vascular ingrowth known as a
glomerulus, and this glomerulus soon grows to such an
extent as to project through the peritoneal funnel, the
neck of which it completely fills, into the body-cavity
(Fig. 66, gl). There is thus formed a series of glomeruli
belonging to the anterior Wolffian tubuli projecting
freely into the body-cavity. These glomeruli with
their tubuli become however early aborted.

THE THIBD DAT.

Fio. 66.

[CBU.

! THBOnOH TEE ExTEKNAL GlOUKKULDS OF ONI
TBE AnTBBTOR SEGME^TAL TUBES OF AX EUBBTO Chici

OP ABOUT lOOHoras.
ifl. glomerulus ; ffe. peritoneal epithelium ; Wd. Wolffian
ao. aorta ; nw. mesentery.
The Wolffian tubule, and the conneution between the
aad internal porta of the glomerulus are not shewn in thia i

In the case of the remaining tubules developed
the S-shaped cords, the attachment to the peiit
epithelium is very soon lost. The cords acquire
lumen, and open into the Wolffian duct. Their bliw
extremities constitute the commencements of M»lpt
ghian bodies.

In the posterior part of the Wolffian body of
chick the intermediate cell-mass becomes very e
detached from the peritoneal epithelium, and at a <
aiderably later period breaks up into oval veaiclea,^
elongate into Wolffian tubules. In addition to
primary tubules, whose development has just
described, secoodary and tertiary tubules are
on the dorsal side of the primary tubuies, Tbtlj

vl] the wolffian body. 193

dififerentiated out of the mesoblast of the intermediate
cell-mass and open independently into the Wolffian *
duct.

A tubule of the Wolffian body typically consists of the follow -
lug parts, (1) a section carrying the peritoneal opening, and
known as the peritoneal funnel, (2) a dilated vesicle into which
this opens, (3) a coiled tubulus proceeding from (2), and termi-
nating in (4) a wider portion opening into the Wolffian duct.

In the chick, the peritoneal funnel is only found in the most
anterior tubules and soon atrophies ; it is not developed in the
tubules of the posterior part of the Wolffian body. Region No.
4 also is not clearly marked o£f from region No. 3. One part of
the wall of the dilated vesicle (2) is invaginated by a bimch of
capillaries and gives rise to the Malpighian body.

In consequence of the continual folding in of the
somatopleure and especially of the splanchnopleure, as
well as owing to the changes taking place in the meso-
blastic somites, the Wolffian duct undergoes on the
third day a remarkable change of position. Instead of
lying, as on the second day, immediately under the
epiblast (Fig. 34, TT.cf), it is soon found to have appa-
rently descended into the middle of the intermediate
cell-mass (Fig. 64, w,d) and at the end of the third day
occupies a still lower position and even projects some-
what towards the pleuroperitoneal cavity. (Fig. 65,
Wd.)

The chief events then which take place on the third
day are as follows :

1. The turning over of the embryo so that it now
lies on its left side.

2. The cranial flexure round the anterior extremity
of the notochord.

THE THIBD DAT, [CHAP. VL

3. The completion of the ciiculatioD of the yolk-
sac; the increased curvatore of the heart, and the
demarcatioii of its aeveial parts ; the appearance of new
aortic arches, and of the cardinal veins.

4. The formation of four visceral clefls and five
visceral archea

5. The involution to form the lens, and the forma-
tion of the secondary optic vesicla

6. The closing in of the otic vesicle.

7. The formation of the nasal pits.

8. The appearance of the vesicles of the cerebral
hemispheres ; the separation of the hind-brain into cere-
bellum and medulla oblongata.

9. The definite establishment of the cnmial and
spinal nerves as outgrowths of the central nervous
syBtem.

10. The completion of the fore-gut and of the
hind-gut; the division of the former into oesophagus,
stomach and duodenum, of the latter into large intestine
and cloaca.

11. The formation of the lungs from a diverticolnm
of the alimentary canal immediately in front of the
stomach.

12. The formation of the liver and pancreas: the
former as two diverticula from the duodenum, which
subsequently become united by nearly solid outgrowths ;
the latter as a single diverticulum also from iii& duo-
denum.

13. The changes in the mesoblastic somites and
the iipptarance of the muscle-plates.

14. The definite formation of the Wolffian bodies
and the change in position of the 'Wolffian duct

CHAPTER Vn.

THE CHANGES WHICH TAKE PLACE DURING THE

FOURTH DAY.

On opening an egg in the middle or towards the end
of the fourth day, a number of points in which progress
has been made since the third day are at once apparent.
In the first place, the general growth of the embryo has
been very rapid, so that its size is very much greater
than on the previous day. In the second place, the
white of the egg has still further diminished, the em-
bryo lying almost in immediate contact with the shell
membrane.

The germinal membrane embraces more than half
the yolk, and the vascular area is about as large as a
halfpenny.

Corresponding to the increased size of the embryo,
there is a great increase in the quantity of blood circu-
lating in the vascular area as a whole, though the sinus
terminalis is already less distinct than it wajs.

The amnion becomes increasingly conspicuous. It
is now seen as a distinct covering obscuring to a certain
extent the view of the body of the chick \»\ift».>JcL^«KA

196 THE FOURTH DAT. [CfllP.

all traces of the JuuctiDii of its folds are by this time
lost. As jet there is very little fluid in the anudotic
sac jwoper, so that the true amnion lies close upon lis
embryo.

The folding off of the embryo from the yolk sm W
made great progress. The splanchnic stulk, which on
the third day waa still tolerably wide, inasmuch asftbout
one third of the total length of the alimentary caoil
waa as yet quite open to the yolk sac below, now be-
comes so much conati'icted by the progressive closing in
of the splanchnopleure folds, that the alimentaiy aid
may be said to be connected with the yolJt sac byaTCty
narrow neck only. This remnant of the splanchnic
atalk we may now call the vitelline duct; though nam)*',
it is as yet quite open, affording still free communic*'
tion between the inside of the yolk aac and the interioi
of the alimentary canal.

The somatic stalk, though narrowing somewhat, i>
much wider than the splanchnic stalk, so that a «id*
fliderable ring-shaped space exista between the tvro,

Another very prominent feature is the increaK iu
the cranial flexure. During the third day, the axis v
the &ont part of the head was about at right angles t<
the long axis of the body; the whole embryo being stiL
somewhat retort-shaped. On this day, however,
flexure has so much increased that the angle betweffl
the long axis of the body and that of the front
of the head is an acute one.

The tail-fold, which commenced to be notioeabb
during the third day, hiLS during this day increased vei;
much, and the somewhat curved tail (Fig. 67) fonr
quite a conspicuous feature of the embiyo, Tl

A TRAK8PARENT OBJECT.

">« Mmion has been completely removed, the cut end of the
~~'~ italic IB shewn at S.3. with tha Blluntoia lAl.) protruding

' Webral horaiBphore. F.B. fore-brain or veaicla of the third
""Wcle (thalftmencephalon) with the pineal gland (Pn.)
^•ctingfrom itaBiimmit Jf.B. mid-brain. (76. cerebelluio.
f^ ■ fourth ventricle. L. lens. cA.s. choroid alit. Owing to
™* SWwth of the optic cup the two lajera of which it is oom-
f _ *« eannot anj longer be seen from tlie surface ; the pos-
7™*" ■arface of the choroid layer alone is visible. Cen. V,
"~'*Wy TBHJcle. j.Tn. superior maiillar J process. lF,%F,(Aa.
^y ■■oeod, third and fourth visceral folds. F. fifth nerve
One branch to the eye, the ophtltalmli: branch, and

€ THE FOUIITH DAT. [CHAP.

anotlier to tlie first visceral arch. VJI. seTentli nerve paBsirig
to the aacood visceral arch. O.Ph. gloBaopharjDgeal nerve
passing tuvraida the third visceral arch. Pff, pncumogastric
nerve paaaing towftrJs the fourth Tisceral arch. iv. investing
mosa {basilar plate). No attempt has been made in the figure
to iodiuate the ptieition of the dorsal wall of tlie throat, which
cannot be easily made out in the living embryo, c/t, noto-
ohord. The front end of this cannot bo neon in the living
embryo. It does not end however us shewn in the figure,
hut takes a sudden bend downwards and then terminatea ia
apotot. Jlc. heart seen through the walla of theuheat. M.P.
muscle- plates. W. wing. ff.L, hind limb. Beneath the
hind limb h seen the curved tail.

curvature of the body has also gone on increasing, and
as the result of these various flexures, the embryo has
become somewhat spirally curled up on itself {Fig. 67).

The distinct appearance of the limbs must be
reckooed as one of the most important events of the
fourth day.

Owing to the continued greater increase of depth
than of breadth, the body of the embryo appears in
section (Fig. 68) higher and relatively narrower than
even on the third day, and the muscle -plates, instead of
simply slanting downwards, come to be nearly vertical
in position. Not far from the line which marks their
lower ends, the somatopleure, almost immediately after
it diverges from the splancbnopleure, is raised up (Fig.
68, W.R.) into a low rounded ridge which runs along
nearly the whole length of the embryo from the neck
to the tail.

It is on this ridge, which is known as the Wolffian
ridge, that the limbs 6rst appear as flattened conical
bads projecting outwards. They seem to be local de-

■. pooteriiir root of epioal norve with gan-
liiotL a.r. anterior root of epinol nerve. A.O.C. anterior
P^ column of Bpinal cord. A. W.C. anterior wtut« coInmD
rfa[nnal cord just commencing to be formed, and nrit ver;
dM&ictly marked in the figure, m.p. muncle-plat«. eA.
Botoohoid. W.R. Wolffian ridge. A.O. donal aorta, r.co.
^ariHenOT cardinal ran. W.d. Wolffiui dud. W.h. Wolffian
body, orauusting of tubules and lialpi^iian corpuBulee. One
' tba latter ia rqceaeuted oo e«oh aide. g.t. gwmiual

I
I

200 THE FOURTH DAY., [CHAP.

epithelium, d. nliuentarj canal. M. commeacing me-
Bentarj'. S.O. Bomutoplcnire. S.P. spluacbnopleum V.
blood-vessels, pp. pleuroperitonenl cavity.

velopments of the ridge, the rest of which becomes lesB
and less prominent aa they increase in size. Each bud,
roughly mangular in section, consists of somewhat
dense mesoblast covered by epiblast which ou the sum-
mit is thickened into a sort of cap. The front limbs or
nings (Fig. 67) arise just behind the level of the heart,
and the hind limbs in the immediate vicinity of the
tail The first traces of them can be seen towards the
end of the third, but they do not become conspicuous
till the fourth day, by the end of which the two pairs
may be already distinguished by their different shapes.
The front limbs are the narrowest and longest, the hind
limbs being comparatively short and broad. Both are
flattened from above downwards and become more so aa
their growth continues.

In the head, the vesicles of the cerebral hemispheres
are rapidly increasing in size, their growth being enor-
mous aa compared with that of the thalamencephalon or
vesicle of the third ventricle. The mid-brain is now, a£
compared to the other parts of the brain, larger than at
any other epoch, and an indistinct median furrow on its
uppei surface indicates its division into two lateral
halves. The great increase of the meaoblaatic contents
of the secondary optic vesicle or involuted retinal cup
causes the two eyeballs to project largely from the sid^
of the head (Fig. 69, Op). The mass of mesoblast which
invests all the various parts of the brain, is not only
growing rapidly below and at the sides, but is also
uadeTgohig developments which result in the formation

A. Hiu) I

Eit&RTo Cbick of the Foubth Di.t
TEWED FROM flELow ^s AN OPAQUE OBJECT. (Chroinic

»dd preparation.)
*■£ cerebral hemiBpheres. F.B. vesicle of the third ventricle

w thai amenoephaloD. Op. ajebatl. nf. naso-froDtal process.

M. oftvity of mouth. S.M. superior nutiill&rj' process of F. 1,

the ant visceral fold (maudibular arcb). F. 2, F. 3 second

*ad third visceral archea. X. oasal pit.

Id Oder to gain the view her« given the neck was cut acron

Km the third and fourth viBceral folds. In the section »
WU msde are seen the alimentarj canal al, the neural canal n.c,
WDntochcird th., the dorsal aorta JO,, and the jugular veina V.
Jfthilbus arterioaas.

In th« drawing the na«al groove has been rather exaggerated
'" <■< upper part. On the other band the lower and BhuUower
^ of the gri-ove whore it runs between the superior maiillary
the broad naso-frontal process has not been
*iifcctorily rendered. Hence the end of the snjierior maxillary
'P^Mv soems to join the inner and not, as described in the text.
"* citw margin of the nasal groove. A few hours later the
^PWtJftn of the two would have been very visible.
4 The same seen sideways, to shew the visceral folds, ot. otio

nnale. Remaining lotl^^rs as before.

202 THE FOURTH DAY. [CHIF.

of the primitive skull. All these eveata, added to llie
cranial flexure spoken of above, give to the antaiK
extremity of the embryo a shape which it becomes nwM
and more easy to recognize as that of a head.

Meimwhile the face ia also beiug changed. TheWi
oasal pits were on the third day shallow depreaaions com-
plete all round. As the pits deepen on the fourthdij
by the growth upwards of a rim round them, a deficiencj
or break in the ridge may be observed on that sideofit
turned towards the mouth; which constitutes a kind rf
shallowgroove(Fig. 60 A^ directed obliquely do wnwaris
towards the cavity of the mouth. The fronto-ttanl
process or median ridge (Fig. CU, nf). which on the thin
day rose up between the superficial projections caused bj
the bulging anterior extremities of the vesicles of the
cerebral hemispheres, and on the fourth day beOHU*
increasingly prominent, separates the two grooves frooi
each other, and helps to form the inner wall of ead) 9
them, white the depth of the groove also becomea in-
creased by the prolongation along its inner side of ^
rim surrounding the nasal pit. Abutting on the onta
side of each groove near the mouth and so helping
form the outer wall of each, lie the ends of the superiof
maxillary processes of the first visceral arch (Fig. 69 R
SM), which like the fronto-nasal process are increasmg
in size. By their continued growth, the groove is
and more deepened, and leading as it does from tbt
nasal pit to the cavity of the mouth, may already ^
recognized as the rudiment of the passage of tha
terior nares.

During the latter half of the fourth day there if-
pears at the bottom of the deep lozenge-shaj * "^

THE CRANIAL NERVES. 203 J

ItbefiUimodaBum or primitive buccal cavity, in the uow ^
'n ira!] dividing it from the alimentary canal, a Ion
ItBiul, or according to Gotte e, vertical slit which, soon ,
Mming a wide opening, places the two cavities in I
^ete coannaoication.
The cavity of the mouth, being, it will be remember-
l formed partly by depression, partly by the growth
Hie surrounding folds, ia lined entirely with epiblast,
I which the epithelium of it« surface and of its
WIS glands is derived. In this respect, as Remak
ited out, it differs from all the rest of the alimentary
il, whose whole epithelium is formed out of hypoblast.
I By the aide of the hind-brain the cerebellum, through
i increased thickening of its upper walls, is becoming
i and more distinct from the medulla oblongata;
&e both in front and behind the auditory vesicle,
Irhich the rudiments of the cochlea and recessus ves-
li are already visible, the cranial ganglia and nerves
lacqoiriBg increased distinctness and size. They may
very plainly seen when the head of the fresh embryo
nilgected to pressure.

The foremost is the iifth cranial nerve (Fig. C7, V.)
bits Oasserian ganglion; it lies a little way behind
I Ulterior extremity of the notochord immediately
iwthe cerebellum. Next to this comes the seventh
re (Fig 67, VTI.), starting just in front of the ear-
de, and extending far downwards towards the second
end arch. The two nerves which lie behind the ear-
licle are now obviously separate from each other; the
*t one ia the glossopharyngeal (Fig. 67, G.Ph.), and
Bttinder one already shews itself to be the pneumo-
o(Fi«. 67,Pj;.).

204 THE FOURTH DAY. [CEIF,

The mesoblastic somites, which by the continued
differentiation of the axial mesoblast at the twl end of
the embryo have increased in number from thir^ to
forty, undergo during this day changes of great import-
ance. Since these changes are intimately conneclrf
with the subsequent development of the verteW
column, it will perhaps be more convenient to describt
briefly here the whole series of events through which
the somites become converted into the pennwMOt
structures to which they give rise, though many of the
changes do not take place till a much later dat« tbu
the fourth day.

The separation of the muscle-plates (p. 187) left tl"
remainder of each somite as a somewhat triangulM
mass lying between the neural canal and notochord
the inside, and the muscle-plate and intermediate erf
mass on the outside (Fig. fii). Already on the tlurld^
(Fig, Co) the upper angle of this triangle grows upwai4
between its muscle-plate and the neural canal,
meeting its fellow in the middle line above, form* i
roof of mesoblast over the neural canal, between it i
the superficial epiblaat. At about the same time,!
inner and lower angle of the triangle grows inwafdfl
wards the notochord, and passing both below it (betw
it and the aorta) and above it (between it and ti
neural canal), meets a similar growth from its fell
somite of the other side, and thus completely inw
the notochord with a coat of mesoblast, which, as sea
Fig. 68, ia at first much thicker on the under than
the upper side.

Both neural caual and notochord are thus fomukB
from neck to tail with a complete investment of

til] THB FEMMASEST YZBTEBSjK. i05

blasts still maiked, however, by the traD5j<iient lines
indicating the {ore and aft limits of the several somite&
This mesoblastiG investment is sometimes spoken of as
the "membranons" vertebral colunm.

The portions of the somites thus forming the primary
vertebrae or membranous vertebral column are converted
into the permanent vertebrae; but their conversion is
complicated by a remarkable new or secondary segmen-
tation of the whole vertebral column.

On the fourth day, the transparent lines marking
the fore and aft limits of the somites are still distinctlv
visible. On the fifth day, however, they disappear, so
that the whole vertebral column becomes ftised into a
homogeneous mass whose division into vertebrae is only
indicated by the series of ganglia. This ftision, which
does not extend to the muscle-plates in which the
primary lines of division still remain visible, is quickly
followed by a fresh segmentation, the resulting segments
being the rudiments of the permanent vertebrae. The
new segmentation, however, does not follow the lines of
the segmentation of the muscle-plates, but is so eflFectod
that the centres of the vertebral bodies are opposite the
septa between the muscle-plates.

The explanation of this character in the segmentation is not
difficxilt to find. The primary segmentation of the body is that
of the muscle-plates, which were present in the primitive forms
in which vertebrse had not appeared. As soon however t^a the
notochordal sheath was required to be strong as well as flexible,
it necessarily became divided into a series of segments.

The condition under which the lateral muscles can best cause
the flexure of the vertebral colunm is clearly that each muscle-
plate shall be capable of acting on two vertebrso ; and this con-
dition can only be fulfilled when the muscle-segments are o()\}o-

THE FOURTH DAT. [CHAP.

site the intervals between the vertebne. For this reason, when
the vertebrEe became formed, their centres were opposite not the
middle of the muacle- plates but the inter- mufteiilar septa.

These oonaiderfttions fallj explain the characters of the
seoondarj segmentation of the vertebral column. On the other
hand the primary segmentation of the vertobral rudiments ia
olearlj a remniiut of a condition when no vertebral bodies were
, present ; and has no greater morphological aigniiioance than the
fact that the cells of the vert«hrtB were derived from the Beg-
nieuted muacle-platea, and then became fused into a oontinuoiu
aheath around the notochord and nervous aiis ; till flnallj they
beonme in still higher forms differentiated into vert«brra and
their arches.

By these changes this remarkable result ia brought
about, that each permanent vertebra ia formed out of
portions of two consecutive mesoblastic aomitea. Thus,
for instance, the tenth permanent vertebra is formed
out of the hind portion of the tenth somite, and the
&0Qt portion of the eleventh somite.

The new segmentation is associated with or rather is
caused by histological changes. At the time when
tiie fusion takes place, the mesoblaat, which in the form
of processes from the somites surrounds and invests
the notocbon!, has not only increased in mass but also
has become cartilaginous, so that, as Gegenbaur' points
out, there is present for a short period on the fifth day
a continuous and unsegmented cartilaginous investment
of the notochord.

This cartilaginous tube does not however long re-
main uniform. At a series of points corresponding in
number to the original aomitea it becomes connected

' ITfOeriJidaini) iur vtrgUuihenden Aiiatomie der WirbtUXnU fret
AmphibitK imd Repcilieit, Leipzig, 1S62.

PERMANENT VERTKBRA:. 207"

with a number of cartilaginous arches which appear iii
the mesoblastic investment of the neural canal. These
irehes, which thus roof in the neural canal, are the
twtilaginous precursors of the oaseous vertebral arches.
We further finti that the portions of the cartilaginoua
tube from which the arches spring come to diiler histo-
l^icaUy from the portions between them not connected
with arches : they are clearer and their cells are less
closely packed. There is however at this period no
diadnct segmentation of the cartilaginous tube, but
oerelr a want of uniformity in its composition.

The clearer portions, from which the arches spring,
"On lilt bodies of the vertebrce, the segments between
wan the intervertebral regions of the column.

Od the fifth day a division takes place of each of the
*lwwrt«^rai segments into two parts, which respec-
"Wly attach themaelvea to the contiguous vertebral
•spoils, A part of each intervertebral region, immedi-
ately adjoining the notochord, does not however undergo
uii) (iinaion, and afterwards gives rise to the ligamen-

n auapensorium.

This fresh segmentation is not well marked, if in-
**(i it takes place at all in the sacral region.

To recapitulate: — the oiiginai somites lying side by
'■we along the notochord, after giving otf the muscle-
P*t«B. grow around, and by fusing together completely
■'at, with mesoblaat, both neura! canal and notochord.

This investment, of which by reason of its greater
Powth the original bodies of the somites now seem to bc
•"y an outlying part, becomes cartilaginous in such a
**y that while the notochord becomes surrounded with
•iick tube of cartilage bearing no signs of segmenta-

208 THE FOURTH DAY. [CHAP.

tioii, but having the ganglia lodged on its exterior at
intervals, the neural canal ia covered in with a scries of
cartilaginous archea, coonecled to each other by ordinary
mesoblastic tissue only, but passing at their bases di-
rectly into the cartilaginous tube around the notochord.

By a process of histological differentiation the carti-
iaginous tube is divided into vertebral and interverte-
bral portions, the vertebral portions corresponding to
the arches over the neural canal. Fresh lines of seg-
mentation then appear in the intervertebral portioafl,
dividing each of them into two parts, of which one at-
taches itself to the vertebra in front and the other to
the vertebra behind.

The notochord. Meanwhile from the fourth to the
sixth day important changes take place in the notochord
itself.

On its lirst appearance the notochord was, as we
have seen, composed of somewhat radiately arranged
but otherwise perfectly typical mesoblast-cella.

On the third day some of the central cells become
yacuoiated, while the peripheral cells are still normal
The vacuolated cells exhibit around the vacuole a peri-
pheral layer of granular protoplasm in which the nucleus
lies embedded, whilst the vacuoles themselves are filled
with a perfectly clear and transparent material, which
in an unaltered condition is probably fluid. Towards
the end of the day the notochord acquires a delicate
atnicturclesa sheath which is no doubt a product of it-s
peripheral cells.

On the fourth day all the cells become vacuolated
with the exception of a single layer of flattened cells at
the periphery. The vacuoles go on enlarging until

^ILJ THE NOTOCHORD. 209

on the siitli (lay the vacuoles in each cell have so much
■ocreased at the expense of the protoplasm that only a
'■eiy thin layer of the latter is left at the circumfertmce
of the cell, at one part of which, where there is gene-
f*lij more protoplasm than elsewhere, the starved re-
IMU13 of a nucleus may geuerally be detected. Thus
"** whole notochord becomes transfonned into a spongy
reticulum, the meshes of which correspond to the vacu-
''lea of the cells and the septa tn the remains of their
odl-walls.

The notochord is on the sixth day at the maximuin
0' its development, the change which it henceforward
'I'ldeigoes being of a retrograde character.

From the seventh day onward, it is at various points
encroached upon by its investment. Conatrictious are
thus produced which first make their appearance in the
intervertebral portions of the sacral region. In the cer-
"ical region, according to Oegenbaur, the intervertebral
i'iOious are not constricted till the ninth day, though in
ihe vertebral portions of the lower cervical vertebrie cou-
'Wcliona are visible as early as the seventh day. By
Mie ninth and tenth days, however, all the interverte-
oral portions have become distinctly constricted, and at
"le Banie time in each vertebral portion there have also
'Ppeartd two constrictions giving rise to a central and
"' two terminal enlargements. In the space therefore
'^'''esponding to each vertebra and its appropriate in-
'^'^ertebra! portion, there are in all three constrictions
"'^ three enlargements.

On the twelfth day the ossification of the bodies

"' the vertebne commences. The first vertebra to ossify

" the second or third cervical, and the ossification gradu

t F. 4 B. \%

■_J

3. iiiia^H

Tee fourth day. [cha^

ally extends backwards. It does not commence in tin
arches till somewhat later. For each arch there ar<
two centres of ossification, one on each aide.

The notochord persists for the greater part of foete
life and even into post-foetal life. The larger vertebrsKi
portions are often the first completely to vanish, The;
would seem in many cases at any rate (Gegenbaur) b
be converted into cartilage and so form an integral part
of the permanent vertebrEB. Rudiments of the
vertebral portions of the notochord may long be detect^
in the ligamenta suspensoria.

We may remind tho reader that in the adult
between each of the vertebns of a nectc and back a
disc — the meniscufl — which is pierced ia the centre,
are thick at the circumference but thin off to a fine edge
the central hole. Owing to the shape of theeo discs there are loft
between each pftir of rertebrai two cavities, which only oonuBU-
nicate througii the central aperture of the meniscus. Thiwigl"
this central aperture there ptusses a band called the ' Ii{
turn Huspensorium,' connecting the two vertebrie.

In the tail the menisci are replaced by bodies known
'aanuli fibrosi,' which precisely resemble the eimilorly naiarf
l-odies in mammftla. They differ from the menisci in beiag
attached over their whole Burface to the ends of the vertebnt
bodies, so that the cavities between the menisci and the vertobw
do not exist. They are pierced howcTer by a body oon»-
aponding with the ligaraeiitum auspensoriiim and known as ll*
'nucleus pulpoHus."

In the intervertebral regions the chorda, soon after the tw*"
menceraent of osaification, entirely disappears. The oartUl^
around it however becomeH converted (in the region of the W*"
into the ligamentum BuspenBorium, which unites the
tebrae between which it is placed.

In the tail the carJilagB becomes the nucleus pulposus, wbic«
corresponds exactly to the ' ligamentum suBpenBorium ' of t**.
neck and bock.

yil] the kuscle-flatbs. 211

Shorilj after the fomiation of the ligamentum suspenflorium
the ramaioiDg cartUage of the intenrertebral eegmenta is con-
Terted into the meoiBCus between each two yertebne, and in the
tail into the annulns fibrosus. Both axe absent in the eaoram.

Kiucle-plates. We shall conclude our account of
the mesoblastic somites by describing the changes which
take place in the muscle-plates.

In the chick these are somewhat complicated, and
have not been folly worked out

On the third day the muscle-plates end opposite the
point where the mesoblast becomes split into somato-
pleure and splanchnopleure. On the fourth day how-
ever (Fig. 68 mp.) they extend a certain distance into
the side walls of the body beyond the point of the
division into somatopleure and splanchnopleure.

Into what muscles of the trunk they become con-
verted has been somewhat disputed. Some embryolo-
gists have stated that they only form the muscles of
the back. We have, however, little doubt that all the
episkeletal muscles, to use Professor Huxley's term
{Vertebrates, p. 46), are their products; a view also
adopted by Professors Huxley and Eolliker.

The development of the subvertebral system of musclen
(hyposkeletal of Huxley) has not been worked out, but on the
whole there is reason to believe that it is derived from the
muscle-plates. EOlliker, Huxley and other embryolc^sts believe
however that these muscles are independent of the muscle-plates
in their origin.

Whether the muscle of the diaphragm is to be placed in the
same category as the hyposkeletal muscles has not been made oa^

It is probable that the cutaneous muscles of the trunk
derived from the cells given off from the muscle-plates. KOl
however believes that they have an independent origin.

THE FOURTH DAY. [L'HAJ'.

The limb-tniuclea, both eitriiiHlo and iiitriusic, are ia certain
fiabes (ElBamobroBcbii), derived from the muscle-plates, and a
Bitnilar origin has been observed in Lacertilia and Amphibia.

In the Chick and other higher Vortebrata on the other hand
the entrance of tbe muscle-plates into the limbs has not been
made out (KOlliker). It seems therefore probable that by an
embiyological modification, of which instaDcea are bo frequent,
the cells which give rise to the muscles of the limbs in the higher
Vertobrata can do longer be tiaced into a direct connectioa with
the musclo-plates.

At first, as is clear from their mode of origin, the
muBcla-plates correspond in number with the protover-
tebrae, and this condition is permanent in the lower
vertebrates, such as fishes, whiTC we find that the
lateral muscle is divided by septa into a series of
segments corresponding in number with the vertebrae.

Wolffian body or meBonephroB. Of all the events
of the fourth day, none perhaps are more irapoitant than
those by which the rudiments of the complex urinary
and generative systems are added to the simple Wolffian
duct and body, which up to that time are the sole repre-
sentatives of both systems.

We saw that the duct arose on the second day (pp.
94, 106) as a solid ridge which subsequently became a
tube, lying immediately underneath the epiblast above
the intermediate cell-mass, close against the upper and
outer angles of the somites, and reaching from about
opposite to the seventh somite away to the hinder end
of the embryo.

At first the duct occupies a position immediately
underneath the superficial epiblast, but very soon after
its formation the growth of the somites and the changes
trbicb take place in the intermediate cell-mass, together

TBE WOLFKiAN BODY.

"tta the general folding in of the body, cause it to
iippear to change its place and travel downwards (p.
'"O- While the shifting is going on, the culls lining
tfle upper end of the pleuroperitoneal cavity (the kind
*" '*a.j which, aa seen in sections, is formed by the diver-
l^c«; of the somatoplenre and splanchnopleure) become
wluijiiiar^ and constitute a distinct epithelium. This
*pit.l»eliiim, which is clearly shewn in Fig. 64, and is
"** indicated in Fig. 65, is often called the gei-minul
'P*^^»^ium, because some of its cells subsequently take
t**^ in the formation of the ovary. Soon after the ap-
l«a*-«ice of the germinal epithelium, the intermediate
f*^— JnasB increaaes in size and begins to grow outwards
^^'^> the pletiroperitoneal cavity, as a rouuded projection
"'*ich lies with its dorsal surface towards the soniato-
P"^*re. and its ventral surface towards the splanchno-
P^Xire, but is in either case separated from these layers
^ a narrow chink. The Wolffian duct (Figs. 65, 68,
^^t)tjavela down, and finally before the end of the third
'*'y is found in the upper part of this projection, near
^'^•'t fcce of it which is turned towards the snmatopleure.
The tubules of the anterior part of the Wolffian
^^'S.y have by the end of the fourth day almost entirely
****^peared; but the tubules of that part of the Wolf-
^*'"*a body which is found behind the 16th segment
**^ergo a further development.

Each increases in size especially that portion which
P^'^^oeeda from the Malpighian body and is known as the
***ile(l tubulus (region No. 3, see p. 103), This becomes
T^^ioh elongated and twisted. As a consequence of this
"^Crease in size the intermediate cell-mass comes to
f**J6ct more and more into the pleuroperitoneal cavity.

n* THE FOURTH DAY. [cHAP.

Tbo large size of the hindtir part of tho Wolffian body aa
compared with that of the anterior part is due to the presence of
the doreally placed Becondary tubules, whoi^e developnieot was
mentioned on p. 192. These are more numerous in the posterior
than in the anterior port of the Wolffian body. At the hied end
of the Wolffian bod j there ore as many as four to, each primary
tubule.

The tubules, which from their contorted course are
in sections (Figs, 68, 71) seen cut at various angles,
possess an epitheUum which is thicker than that of the
Wolffian duct. From this diiFerence it ia generally easy
to distinguish the sections of the tubules from those of
the duct. The glomeruli of the Malpighian bodies are
in sections of hardened embryos usually filled with
blood -corpuscles.

Towards tho hind end of the embryo, the projection
of the intermediate cell-mass spoken of above becomes
smaller and smaller, and the Wolffian duct is thus
brought nearer to the splancbnopleure, and in the
region of the hind-gut comes t« lie close to the walls of
the alimentary canal. On the fourth day, the two ducts
meet and open into two horns, into which the side-walls
of the recently formed cloaca are at that time produced,
one on either side. '

As wo shall afterwards see, the ducts of the perma-
nent kidneys and Muller's duct also fall into these two
horns of the cloaca.

The Wolffian bodies thus constituted perform the
offices of kidneys for the greater part of embryonic life.
In the chick they disappear before birth; but in most
of the Ichthyopsida they remain for life as the perma-
nent kidneys.

JfiiUflnaJl duct. After the establishment of the

MULLEBIAN DID

WolSaa body there is fomecl in both sexes a duct,
waicn in the female becomea the oviduct, but which in
tlw male is fiinctionless and usually disappears. This
onct, in spite of certain peculiarities in its development,
H without doubt homologous with the UulleriaQ duct
of the Ichthyopsida.

The first rudiment of the Mullerian duct appean at
tMMid of the fourth day, as three successive open invniu-
tiwis of the peritoneal epithelium, connected together
h more or less well-defined ridge-like thickenings of
"le epithelium. It takes its origin from the layer of
widtened peritoneal epithelium situated near the dorsal
•ngle of the body-cavity, close to the Wolffian duct, and
xiine considerable distance behind the front end of the
Wolffian duct.

In a slightly later stage the ridges connecting the
PW^ffl become partially constricted off from the peri-
""Wl epithelium, and develop & lumen. The condition
•if tbe Btructure at this stage is illustrated by Fig. 70,
'^'fesenting three transverse sections through two
pooves, and through the ridge connecting them.

The Mullerian duct may in fact now be described aa
'short but slightly convoluted <luct, opening into the
'J^^J-iavity by three groove-like apertures, and con-
™"ied for a short distance behind the last of these.

In an embrjo not very much older than the one last
••criWd the two posterior apertures vamsb and the
^'firior opening alone remains as the permanent opeu-
'"gofrhe Mullerian duct.

The position of these openings in relation to the
"iiffian body is shewn in Fig. 71 , which probrvbly paaaee
""■"Ugh a re^on between two of the peritoneal opening

SE(7riOKS SHEWINU IWO OF THB PbRIIONEAL InVAOINATIONB
WHICH (JIVE RIHE TO THE AsTERlOa PiBT OP THK MCL-

LBRiAX Uuoi (Pronsphros).

A is the llth Bectiou of the series.
6 „ IStb „ „

grS seooad groove, ^3 third grMve
Wolffian duct.

r 2 socoud ridge, icd.

Afl long as the openings persist, the Mtlllerian duct
consists merely of a very small rudiment, continuous
with the hindermost of them, and its solid extremity
appears to unite with the walls of the Wolffian duct.

After the closure of the two hinder openings the
UuUerian duct communcea to grow rapidly backwards,
and for the first part of its subsequent course it
appears to be split off as a solid rod from the outer or
ventral wall of the Wolffian duct (Fig. 72). Into this
rod thtt lumen, present in its front part, subsequently
ext«nda. Its mode of development in front is thus prt'-
cisely similar to that of the Miillerian duct in Elasiuo-
brauchii and Amphibia.

This mode of development only occurs however in
the anterior part of the duct. In the posterior part of

TBS HUIXESIAK UCCT.

or THX lltTEBXtDlATK CCLL-IUBS I
D*T. (From Woldejer.) Hafpiified 160 tunea

L. Bomatopleon. d. portion of the genuinal
from which the involution to form the duct of
UiU]er («) Ucea place, a. thiuk?D«l portion of the germinal
tpitlielium in which the primHiye ova C and o ore Ifing.
B. modified meeobUst which will fonn the stroma of tlu)
oraiy. WK. Wolffian body. y. Wolffian durt.

coune its growing point Uea in a bay formed by the
Wler wall of the Wolffian duct, but does not become
^it*ly attached to that duct It
pMaible that, although not actually split off from

become i

however I

from th^^^J

Two Sbctionb sHEniNQ THE Jdnotion of the Tekuinai,
Solid Poktioh or the MOLtsRUN Duct with tbb
Wolffian Duct.

In A the tennintd portion of the duct ia quite distinct ; in B
it baa united with the walls of the Wolffian duut.

md. Mlillerian duct. Wd. Wolffian duct

walla of the Wolffian duct, it may grow backwards from
cells derived from that duct.

The Miillerian duct finally reaches the cloaca thoui^h
it does not in the female for a long time open into it,
and in the male never does so.

The anterior part of the commencing Miillerian duct with its
three openinga into the body-cavity is probably homologous with
the head kidney or pronephros of the Iciitbyopaida.

Permaaent kidney or metanepbros. Between the
80th and 1 00th hour of incubation, the permanent lad-
aeya begin to make their appearance, and as is the case
with the Wolffian bodies, the first portion of them to
appear ia their duct. Near ita posterior extremity the
Wolffian duct becomes expanded, and from the expand-
ed portioD a diverticulum is constricted oEF which in a

THE PERMANENT KIDNEY.

tonCTerae section lies dorsal to the original duct, and
nd end of which points forwarda, that is, towards
tie head of the chick. This is the duct of the perma-
nent kidney or ureter. At first the ureter and the
Wcilffisn duct open hy a common trunk into the cloaca,
but ttiis state of things laate for a short time only, and
tj the nith day the two ducts have independent open-
ings.

The ureter thus beginning as an outgrowth from
tbe Wolffian duct grows forwards, and extends along
tte outer side of a mass of mesoblastic tissue which
Iwb miunly behind, but somewhat overlaps the dorsal
»»PMI of. the WolflSan body.

This mass of mesoblastic cella may be called the
letaiiephric blastema. It is derived from the interme-
™« cell-maas of the region reaching from about the
wirty-first to the thirty-fourth somite. It is at first
'Wntiauous with, and indistinguishable in structure
'*Wn, the portion of the intermediate cell-mass of the
ftgion immediately in front of it, which breaks up into
Wolffian tubules. The metanephric blastema remains
"Wever quite passive during the formation of the
'"olKan tubules in the adjoining blastema; and on the
knastion of the ureter breaks off from the Wolffian
•odj in front, and, growing forwards and dorsalwards,
womea connected with the inner side of the ureter

the position just described.

In the subsequent development of the kidney col-
Wing tubes grow out from the ureter, and become

■tinuous with luasses of cells of the metanephric

tt«ma, which then differentiate themselves into the

Iney tubules.

THE KOUBTH DAT.

[chap.

The formation of the kidneys takes place before the
end of the seventh day, but they do not become of func-
tional importance till considerably later.

From their mode of development it clearly follows
Uiat the permanent kidneys are merely parte of the
same system na the Wolffian bodies, and that their se-
paiation from these is an oocurrence of a purely second-
ary impurtance.

The generative ridge. Before describing the sub-
sequent fate of the Wolffian and Miillerian ducta, it will
be necessary to give an account of the formation of the
true sexual glands, the ovaries and testes.

At the first appearance of the projection from the in-
termediate cell-mass, which we may now call the genital
ridge, a colmnnar character is not only visible in the
layM of cells covering the nascent ridge itself along its
whole length, but may also be traced for some little dis-
tance outwards on either side of the ridge in the cells
hning the most median portions of both somatopleure and
splanchnopleure. Passing outwards along these layers,
the columnar ceils gradually give place to a flat tease-
lated epithelium. As the ridge continueid to increase
and project, the columnar character becomes more and
more restricted to ceils covering the ridge itself, over
which at the same time it becomes more distinct On
the outer side of the ridge, that is on the side which
looks towards the somatopleure, the epithelium under-
goes, as we have seen, an involution to form the com-
mencement of the duct of Milller, and for some little
time retains in the immediate neighbourhood of that
duct its columnar character (Fig. 71, a'), though even-
tually losing it.

pnyecdob aliht Woffian bcMh-,«aid iiev^M^ <|M^«i6mi
npidlj beoQBoeE ItattPiipn.

Ob iiie inskie cf ilie ndge lM«ine%^er, diat » «» t^ s»^
kwkng towMtk ite ffihfirhfw|ilem^ the f|Bllwal^^
aotj leUms ixs nnhnnnir donctier, bat gims SK'tiiml
oelk deep (Fig. n, a), vliile at the sKne luie tdie 1^^
Uaat (£) mfederiTing it beooiDes thickeaadL In dtt»
way, owbig pardr to tlie incroasing thkfaiefis of th^
epkhe&nn, aod pully to the aooomalalMD of iiMsoKlasi
beneath it, a slight eminenoe is fonnecL which wh«a
viewed fincmi below, after <^peiuiig the abdominal cantjr^
afqpeais in diied light as a fusifonn white pati^ or
streak, in its early stages extending along the whoW
loigth of the Wolffian body and genital ridge^ but sub^
sequently restricted to its anterior porti<m. Its appoai^
anoe under these cirdunstances has been well d<»cribed
by Von Baer.

This 'sexual eminence' is present in the early stages
of both sexes. In both the epithelium consists of several
layers of short cylindrical cells, a few of which are con-
spicuous on account of their size and their possessing a
highly refractive oval nucleus of considerable bulk; in
both, the underljdng thickened mesoblast consists— as
indeed at this epoch it does generally in all parts of tho
body — of spindle-shaped cells.

The larger conspicuous cells of the epithelium
which appear to have quite a common origin with their
fellow cells and to arise from them by direct difforon-
tiation, and which are seen at the first in male as
well as female embryos, are the primordial ova or pri-
mitive germinal cells (Fig. 71, o). Thus in quite oarl^

S22 THE FOURTH DAY. [tHAP,

stages it is impossible to detect the one sex from the
other.

The ovary. In the female the primordial ova en-
large and become more numerous, the whole epithelium
growing thicker and more prominent, and the spindle-
shaped cells of the underlying mesohlast also increase
rapidly and thus form the stroma of the ovary. The
primordial ova after undergoing some further changes,
into which it is not within the scope of this work to
enter, become surrounded by a number of the ordinary
epithelium cells. These form a distinct layer, the folli-
cular epithelium, round the ovum. After a time there
appear numerous vascular ingrowths from the stroma,
which penetrate through all parts of the germinal epi-
thelium and break it up into a sponge-like structure
formed of trabeculffi of genninal epithelium interpene-
trated by vascular strauds of stroma. The trabecule
of the germinal epithelium form the egg-tubes of
PflUger.

Id this way each ovum becomes invested by a cap-
sule of vascular connective tissue lined internally by
a layer of epithelium; the whole constituting a Oraafian
follicle.

The large nucleus of the primordial ovum becomes
the germinal vesicle, while the ovum itself remains as
the true ovum ; this subsequently becomes enlarged by
the addition of a quantity of yolk derived from a differ-
entiation of its protoplasm.

The testis. The first traces of the testes are found
in the dorsal and inner side of the intermediate cell-
mass, and appear about the sixth day. From the first
they differ from the rudimentary ovaries, by coming into

vii] TUB TESTIS. 223

uiuewh&t close coniiectioii with the Wolffian bodies;
but <xsupy about the same limits from before back-
wink The mesoblast in the position we have men-
<m(i b^ina to become somewhat modified, and by
tie ^bth day the testis is divided by septa of connec-
tire tissue into a number of groups of cells; which are
the commencing tubuli serainiferi. By the sixteenth
% the cells of the tubuli have become larger and
Wjiiirod a distinctly epithelial character.

The history of the primordial cells in the male has
not been so thoroughly worked out as in the female.
The spermatozoa appear to arise by the division of
the inimitive ova (present, as we have stated, in the
'Srij stages of both sexes), which probably migrate
I mtv the adjacent stroma, accompanied by some of the
~ lifferent epithelial cells. Here the primitive germi-
m*!^ ceib increase in number and give rise to the cells
g the secretory tubules of the testes.

Outgrowths frwm the Malpighian bodies of the
Wol£Ban body appear to be developed, which extend
lo Uie testis and come into connection with the true
liferous stroma.

It is evident from the above account that the male
"id female generative products are homodynamous,
""t the consideration of the development of the pro-
'I'lcls m the two sexes shows that a single spermatozoon
^ not equivalent to an ovum, but rather that the whole
''1 <he tpermatoioa derived from a priviordial ovum are
''Vt^^er eqftivalent to one ovum.

We have now described the origin of all the parts
'tiich form the urinary and sexual systems, both of the
'^mbryo and adult. It merely romains to speak briefly

224 THE FO0BTH DAY, [CHAP.

of the changes, which on the attainment of the adult
condition take place in the parts described.

The Wolffian body, according to Waldeyer, may be
said to consist of a sexual and urinary part, which can,
he states, be easily distinguished in the just-batcLed
chick. The sexual part becomes in the cock the after-
teetes or coni vasculosi, and consists of tubules which
lose themselves in the seminiferous tubules. In the
hen it forms part of the epoophoron' of Waldeyer, and
is composed of well -developed tubes without pigment.
The urinary part forms in both sexes a small rudiment,
consisting of bhndly ending tubes with yellow pigment ;
it is most conspicuous in the hen. This rudiment
has been called by Waldeyer parepididymis io the male
and paroophoron in the female.

The Wolffian duct remains as the vas deferens in
the male. In the female it becomes atrophied and
. nearly disappears.

The duct of Miiller on the right side (that on the
left side with the corresponding ovary generally dis-
appearing) remains in the female aa the oviduct. In
the male it is almost entirely obliterated on both
sides.

Vascular system. We may return to the changes
which are taking place in the circulation.

On the fourth day, the point at which the dorsal
aorta divides into two branches is carried much further
back towards the tail.

A short way beyond the point of bifurcation, each
vessel gives off a branch to the newly-formed allantois.

1 This ia aim nailed paroTonain (His), and Boaenmiiilei'B orgm.

nt] THE ARTERIAL ARCHES. 225

It ia Qot, however, till the socoad half of the fourth day,

»fieii th'L- allastois grovra rapidiy, that these allantoic,

"F, 33 they are sometimes called, umbilical, arteries

Kqiiire any importance, if indeed they are present

Wore.

The ril«lline art«ries are before the end of the day

LgiTen off from the undivided aortic trunk as a single

lint quickly bifurcating vessel, the left of the two

Ibtisches into which it divides being much larger than

llbe right.

During this day, the arterial arch running in the
rt visceral fold becomes obliterated, the obliteration
ig accompanied by the appearance of a new (fourth)
b nmning in the fourth visceral fold on either

The second pair of arterial arches also becomes

'Hy (if not entirely) obliterated; hut a new pair of
I developed in the last (fifth) visceral fold,
Wind the last visceral cleft; so that there are still
* pairs of arterial arches, which however now run
■ the third, fourth and fifth visceral folds, the last of
Uie« being as yet small. If we reckon in the slight
•''Duna of the second pair of arches we may consider
"ist there are in all four pairs of arches. When the
™taiid second arches are obliterated, it is only the
antral portion of each arch on either side which abso-
"itely disappears. The ventral portion connected with
'"6 bulbus arteriosus, and the dorsal portion which
J'liM the dorsal aorta, both remain, and are both carried
Jtraight forward towards the bead. The ventral }>or-
'"^ of both first and second arches unite on each side
'^fotm a branch, the external carotid (Fig. 73, E.G.A.),
L p. * B. Va

SSU THE FOURTH DAY.

which runs straight up from the bulbua arteriosus
the head.

Fin. 73,

State o? Artkrial Circulation on the Fine or Sixth

Day.

E.C.A. external oarotid. I.C.A. internal carotid. D.A. dorsal
aorta. Of.A. vitelliiie artery. U.A. allantoic arteries.

In the same way the dorsal portions form a branch,
the internal carotid, which takes its origin from the
doisal or far end of the third arch.

In the venous system important changes also occur.

As the liver in the course of its formation wraps
round the common trunk of the vitelline veins, or
meatus venosus, it may be ssdd to divide that vessel
into two parts : into a part nearer the heart which is
■ called the sinus venosus {Fig. 74. S.V.), and into a part
surrounded by the liver which is called the dvcttia
venosus. Beyond, i.e. behind the liver, the ductus veno-
sus is directly continuous with the vitelline veins, or, e
we may now say, vein, for the right trunk has become
BO small as to appear a mere branch of the left (Fig.
7*. 0/.).

VEINS OF THE LIVEK.

Venous Circclatios at tbb Comuksce-
FiFTH Day,

" fieart, d.c ductuH Cuvieri. Into the ductus Cuvieri of each
side bll J. the jugular vein or superior cardinal vein,

T the T^ia from the wing, and c the inferior cardinal vein.
S.V. ainiia venoaua. Of. vitelline vein. U. allantoic vein,
Khich at this stage gives off branches to the bodj-wall&

V.CI. Tena cava inferior. I. XWer.

The hepatic circulation, which waa commenced on
'I'e third day, hccomes completely establiahed. Those
Itancbes which come oflF from the ducttis venoaiis soon
""^r its entrance between the liver lobes carry blood
""o tile substance of the liver and are called verue
"^Khtntes, while those which join the ductus venomis
"ncirtly before it leaves the liver (i.e. nearer the heart)
'^"^ blood away ima the hepatic substance into the
''"ctiu and are called ver^B revehente*. As a result of
'''ia urangement there is a choice of paths for the
"''*d in passing from the Tit^lline vein to the mnUB
"nosus; it may pass through the capillary net-work
^i Ibu liver, going In by the vena; advehentes and
lb— I

SIS THE FOURTH DAY. [CHAP.

coming back again by the vense revehentea, or it may
go straight through the ductus venosus without passing
at all into the substance of the liver.

As the aUmentary canal by its continued closing in
becomes on the fourth day more and more distinct from
the yolk-sac, it gradually acquires veins of its own, the
mesenteric veins, which first appear as small branches
of the vitelline vein, though eventually, owing to the
change in the relative size and importance of the yolk-
sac and intestine, the latter seems to be a branch of
one of the former.

Corresponding to the increase in the size of the
head, the suptirior cardinal veins (Fig. 74, J.) become
larger and more important and are joined by the wing
veins (TF.). As before, they form the ductus Cnvieri
(d.c) by joining with the inferior cardinal veins (c),

The latter are now hirgely developed, they seem to
take origin from the Wolffian bodies, and their size and
importance is in direct proportion to the prominence of
these bodies. They might be called the veins of the
Wolffian bodies.

As the kidneys begin to be formed a new single
median vein makes it^ appearance, running from them
forwards, beneath the vertebral column, to fall into the
sinus venosus (Fig. 7i, V.C.I). This is the vena cava
inferior.

As the lungs are being formed the pulmonary veins
also make their appearance and become connected with
the left side of the auricular division of the heart.

The blood carried to the allantois by the allantoic
arteries is brought back by two veins, which very soon
after their appearance UQit«, close to the allantois, into

™3

: Mk Jua- -at xneliiw -■« ^I^

IW omesn 4 or AixkZ t vilir tt t^ vmtnMM Kn
eone mod liiifter, ■> ffld l^ onTex «r vtatxkl W^
oatiietiani di^.

from the aariciM «o ibe odaa: IV lUMr oMMnclKw
is very diginrt, and ncara ibe bum of ouNiiM Aun'
adari* (fig. 75, C-A.Y, the foniMr. aMDotiuM <«)W«t
the/ntam Halleri, is £u- less conspknovo.

HuBT OF A Chick o» thh Fourth Day or ImiutiATloN

TIEWEO rROV TBI VEHTRAIi HtllirAtlll,

2,0. left aurioular Kpp«Qdago, C.^, oiuinllii ftiirluiilnriii. f, v«lh
trioltt, b. bulbui utarigiui,

230 THE FODRTH DAT. [CHAP.

The most important event is perhaps the formatioa
of the ventricular septum. This, which commenced «
the third day as a crescentric ridge or fold spriiipng
from the convex or ventral side of the roundix! H'D-
trieular portion of the heart, now grows rapidly acruffl
the ventricular cavity towards the concave or dotal
side. It thus forms an incomplete longitudinal par-
tition, extending from the canalis auricularis to the
commencement of the bulbus arterioans, and dividing
the twisted ventricular tube into two somewhat curve**
canab, one more to the left and above, the other to the
right and below. These communicate freely with each
other, above the free edge of the partition, along it*
whole length.

Externally the ventricular ixirtion as yet shews i»<*
sign of the division into two parts.

The bulbus arteriosus (Fig. 75, b.) has increased i**
size, and ia now very conspicuous.

The venous end of the heart is placed still mo*^
dorsal, and to the left of the arterial end ; its walls a*^
beginning to become thicker.

The auricles are nearly if not quite as far forwa*^
as the ventricles, and the auricular appendages (Fi-
75, l.a.), which were visible even on the third day, aJ
exceedingly prominent, giving a strongly marked e
temal appearance of a division of the auricular porti'
of the heart into two chambers ; but Von Baer ^
unable to detet^t at this date any internal aurici*-^
septum.

The chief events then of the fourth day are : —
(1) The increase of the cranial and body

fl^M|fl

VII.] SUMMARY. 2.11

(2) The increase in the tail-fold.

(3) The formation of the limbs as local thickenings
of the Wolffian ridge.

(4) The formation of the olfactory grooves.

(5) The absorption of the partition between the
mouth and the throat.

(6) The vacuolation of the cells of the notochord.

(7) The formation of the ureter.

(8) The formation of the duct of Muller.

(9) The appearance of the primitive ova in the
jenmnal epithelium.

(10) The development of a fifth pair of arterial
arches, and the obliteration of the first, and partial
obliteration of the second pair.

(11) The development of the ' canalis auricularis/
the growth of the septum of the ventricles and of the
aiuicalar appendages.

CHAPTER Vin.

THE CHANGES WHICH TAKE PLACE ON TEE WIS
DAY.

On opening &n pgg about the middle of the fifth
day, the observer's attention is not arrested by any new
features ; but be notices that the progress of develop-
ment, which was so rapid during the later half 0^
the fourth day, is being continued with undiminisbed
vigour.

The allantois, which on the fourth day began W
project from the pleuroperitoneal cavity, has grown vwj
rapidly, and now stretches away from the somatic stalk
far over the right side of the embryo (which it will 1«
remembered is IjTug on ita left side) in the cavity
between the two anmiotic folds (Fig. 9, K.). It i>
very vascular, and ab-eady serves aa the chief organ rf
respiration.

The blastoderm has spread over the whole of tlM
yolk-sac, and the yolk is thus completely enclosed io
a bag whose walls however are excessively delicate and
easily torn. The vascular area extends over about two-
thirds of the yolk.

The splimchnic stalk or vitelline duct haa m*
reached its ^eateat narrowness ; it has become a soW

CHAP. Vm.] THE LIMBS. 233

cord, and will uodergo no further change till near the
time of hatching. The space between it and the so-
matic stalk is still considerable, though the latter is
nazTower than it was on the fourth day.

The embryo remains excessively curved, so much
so indeed that the head and the tail are nearly in
contact.

The limbs have increased, especially in length ; in
each a distinction is now apparent between the more
cylindrical stalk and the flattened terminal expansion ;
and the cartilaginous precursors of the several bones
have already become visible.

The fore and hind limbs are still exceedingly alike,
and in both the stalk is already beginning to be bent
about its middle to form the elbow and knee respec-
tively.

The angles of both knee and elbow are in the first
instance alike directed outwards and somewhat back-
wards. By the eighth day, however, the elbow has
come to look directly backwards and the knee forwards.
In consequence of this change, the digits of the fore
limb point directly forwards, those of the hind limb
directly backwards. This state of things is altered by
a subsequent rotation of the hand and foot on the arm
and leg, so that by the tenth day the toes are directed
straight forwards, and the digits of the wing backwards
and somewhat ventralwards, the elbow and knee almost
touching each other.

While these changes are taking place the differences
between wing and foot become more and more distinct
The cartilages of the digits appear on the fifth day as
streaks in the broad flat terminal expansions^ firom

234 THE FIFTH DAY. [cEtf.

even curved edge of which they do not project On &i
sixth or seventh day the three digits of the wing (^
median being the longest) and the four (or in sw»
fowla five) digits of the foot may be distinguished, tmi
on the eighth or ninth day these begin to project froni
the edge of the expanded foot and wing, the subatawe
of which, thin and more or less transparent, remsofls fori
some time as a kind of web between them. By tl*
tenth day' the fore and hind extremities, save for tbi
absence of feathers and nails, are already veritslA
wings and feet.

Within the mesoblast of the limbs a continuon
blastema becomes formed, which constitutes the fini
trace of the skeleton of the limb. The correspondisj
elements of the two limbs, viz. the humerus and femiH
radius and tibia, ulna and fibula, carpal and taiaJ
bones, metacarpals and metatarsals, and phalanges, be
come differentiated within this, by the conversion of defi
nite regions into cartilage, which probably are at 611
united. These cartilaginous elements subsequently oaai^

The pectoral Eirdle. The Bcapulo-ooracoid elementsoffl
Hhrmlilur girdle are formed as 0, pair of cartilaginous p' '
one on each Bide of the body. The dorsal half of eAoh
ossifies aa the Bcapula, the ventral an the coracoid. The cIk
are probably membrane bones.

The pelvic girdle is derived from a pair of caitilt^
plates, one on each side. Each of them is developed in
tinaity with the femm* of its Bide. The dorsal half of eaob |
ossifies OS the ilium ; the ventral half becomes proloogoil tl
two processes, the anterior of which osHifies na the putria,
]xisterior at the ischium.

Bibe and sternum. The ribs appear to arise
cartilaginous bars in the connective tissue of the b*

THE CRASIUM. 23a

*»lls. Thoy are placed opposite the intervals between
ite mil Bcle -plates, and are developed independently of
IS vertebra, with the transverse processes of which
P^ subsequently become closely united by fibrous

The stemiim appears to be formed from the fusion
tile ventral extremities of a certain number of the
I. The extremities of the riba unite with each other
in before backwards, and thus give rise to two car-
Iginoua bands. These bands become segmented off
ID ihe ribs with which they are at first continuous,
I Bubsequently fuse in the median ventral line to
B the unpaired sternum.

The aknlL Two distinct sets of elements enter into

I coinposition of the avian skull. These are (1) the

taaia proper, (2) the skeleton of the visceral arches.

The craniam. As we mentioned in the last chap-

the formation of the primitive cranium commenced

■ '"n the fourth day. This primitive cranium, in its

"hit stage, inasmuch as it is composed of condensed

''ut otherwise only slightly differentiated mesoblast, may

■* spoken of as the membranous cranium. On the sixth

wy trie hyaline cartilage makes its appearance as a

itiation within the merabranoiis cranium. The

lous cranium is composed of the following parts,

■(1) A pair of eartili^nous plates placed on each

of the cephalic section of the notochord, and known

parachordala (Fig, 76, iv.). These plates, together

_ the notochord (nc.) enclosed between them, form a

^owfor the bind- and mid-brain. The continuous plate,

fimiiud by them and the notochord, is known as the

THE nFTH DAT.
Fio. 78,

[chip.

View from above of the Pahachordals asd oy the TW*
cDLJt OS TUB Fifth Day or Incubatios. (From Parker.)

In ordar to shew thia the whole of the upper portion ot U*
head has been tMixd away. Tho cartilaginous portions of tli'
skull are marked with the dark horiuontal shading.

(!.w. 1. cerebral vesicles (aliced off). «. ej-e. no, notoobiW'
tV. parachordal 9. foramen for the eiit of the nintli
d. cochlea. k.$.e. horisontal semi-circular canal, j. qo*'
rate. 5. notch for the pasaage of the fifth nerve. ^. t^'
panded anterior end of the parachordals. pLL pitaitirf
BpHoe. tr. trabeculte. The reference line tr has ftccideoldy
been made to end a little short of the cartilage.

(2) A pair of bars forming the floor for the fwf
brain, and known as the trabecules (tr.). These ban '
continued forward from the parachordals, with which,
the chick, they are from the first continuous. TJnitw

Vm.] THE PARACHORDALS. 237

behind where they embrace the front end of the noto-
chord, they diverge anteriorly for some little distance and
then bend in again in such a way as to enclose a space
— the pituitary space. In front of this space they again
unite and extend forwards into the nasal region.

(3) The cartilaginous capsules of the sense organs.
Of these the auditory and olfactory capsules unite more
or less intimately with the cranial walls, while the optic
capsules, forming the sclerotics, remain distinct.

The paxachordals and notochord. The first of

these sets of elements, viz. the parachordals and noto-
chord, forming together the basilar plate, is an unseg-
mented continuation of the axial tissue of the vertebral
column. It forms the floor for that section of the brain
which belongs to the primitive postoral part of the
head, and its extension is roughly that of the basiocci-
pital of the adult skull.

Laterally it encloses the auditory sacs (Fig. 76), the
tissue surrounding these (forming the so-called ' periotic
capsules ') is in the chick never separate from the basi-
lar plate. In front it becomes narrowed, and at the
same time excavated so as to form a notch on each side
(Fig. 76, 5) through which the fifth nerve passes ; and
in front of this it again becomes expanded.

In order to render our subsequent account more
intelligible, we may briefly anticipate the fete of the
basilar plate. Behind it grows upwards on both sides, and
the two outgrowths meet above so as completely to enclose
the medulla oblongata, and to circumscribe a hole known
as the ' occipital foramen,* And it is at this point only
that the roof of the skull is at any period formed of
cartilage.

238 THE FIFTH DAY. [CBi?,

It will be convenient to say a few wonla here with reftrwuT
to the Qototihord in the head It always eitenda along tkSw
\ of the mid- and hiud-brains, but endii immediutely briiind tb

K infundibuluin. The front cud of the notochord becomea mmii

H leas ventrallj flexed in cornjapondence with the craniai Benin ;

H ' its ajiterior end being in some animals (Elasniobrancliii] aliwl

■ bent bac:kward3 (Fig, 77).

(

K Tonw

LosQiTCDiNiL Section through the Head of k

pRiBTiDRrs EMBBro.

eer. commenoement of the cerebral hemiaphere. pn. pinMl ^w

In. infandibulum. pt. ingrowth &om mouth to fi

pituitarj' body. tab. mid-brain, cb. eerebellum. A. wHv

chord, al. alimcutar; tract Jaa. artery of manditnW

E&Iliker hna Bhewn that in the Rabbit, and a more or !<■
similar phenomenon may alao be observed in Birds, the Rnltni
end of the notoohord ia united to the hypoblast of the UuMt ^
immediate contiguity with the opening of the pituitMJ h '
but it is not clear whether this is to be looked upon U
remnant of a primitive attachment of the uotochord to tUehjr"
blast, or as a secondary attachment.

Witliin the b.iailar plate the uotochord often eihibits two '
more dilatotiona, which have been legardeJ by Parksr «
KolUker aa indicative of a aegraeotation of this plate j but tfc*
hardly appear to be capable of this inteipretatioi

iijwsFtrrf ±nixL -iu: war^f<mMfr jl iiife jnmraon

the ^iaiDsr isn it -:iK s^shol uc "se *=u«£7xls wn£ii
iMHir '^ac 'iiHafe i«r:i. ol mvc^ ic -iifinir snLLOzrij 9>

'SUB iiifcafsriiaL viuwjol 'zec pc&:ni:c:&ij» «fti «Im
ttaauBuat luuE '^ i>'mi-rrj«f ^^jer^ §a3xm «: *« a? 7?ik$ea j^c»ii»e

thtfs '±B pttrKcr.rfaui ^1% ^isarcie 5:c=aed as ici« mInw of thfd

znAXfjeLLSZA "icit ^n.'.fii'.'rils bkTe :«ec. hilri io\»i^ pair of bnoiolual
bars; vr: itssk ^jsrw his sow own pfWKkl^T ^iriKi ujv Thev
hare &lio Uen R^u^iai as equivalent to a co«ui^eU» fvidr \>f
necrsJ atcIks enTeli-^ir:-? tise firv»nt end of tl» braiiv Th* |>riiui-
tivc extensioa of the base of the foK^bmin thrcm^h tho |utuitArY
space is an argument, not without fcuw, whwh ha« Iww hpjh^aU^I
to in support of this view.

In the majority of the lowor foniw tho tmUnHilw*
arise quite independently of tho i>ttmohimli*|M, thiuigh
the two sets of elements wnm unitt»; whilo in \\\uW
(Fig. 76) and Mammals tho iMimohtmlwU iiiul !r«lMnMilm
are formed as a continuous wholo, Thu Juiu'tloii \w^

S40 THE FIFTH DAT. [CHAP.

tween tlie trabeculte and parachordals becomes marked
by a cartilaginous ridge known as the posterior clinoid.
The trabeculte are somewhat lyre-shaped, meeting in
front and behind, and leaving a large pituitary space
between their middle parts (Fig. 76). Into this space
there primitively projects the whole base of the fore-brain,
but the space itself gradually becomes narrowed, till it
usually contains only the pituitary body. The carotid
arteries paas through it in the embryo ; but it ceases to
be perforated in the adult. The trabeculse soon unite
together, both in front and behind, and form a complete
plate underneath the fore-brain, ending in two horns in
the interior of the frooto-nasal process. A special rer-
tical growth of this plate in the region of the orbit
forms the interorbittU plate (Fig. 78, ps.), on the upper
surface of which the front part of the brain rests. The
trabecular floor of the brain docs not long remain
simple. Its sides grow vertically upwards, forming a
lateral wall for the brain, in which two regions may
be distinguished, viz. an aiisphenoidal region (Fig. 78,
as) behind, growing out from what is known as the
hasisphenoidal region of the primitive trabeculie, and
an orbitosphenoidai region in front growing out from
the pretphenoidal region of the trabecule. These
plates form at first on each side a coDtinuous lateral
wall of the cranium. At the front end of the
brain they are continued inwards, and more or less
completely separat* the true cranial cavity from the
nasal region in front. The region of the trabeculae in
front of the brain is the ethmoidal region ; it forms the
anterior boundary of tbe cranial cavity. The basal part
of this region forms an intemasal plate, from which an

"«1

ifnTTmir;»fT bek>v ^hia is the abaefUl cMrtD^v. «M. •*!)•
moid. pp. pais pLSiB. ^ prespbrodid or intMHirbitlil.
pa. pilMine. pg. ptcTTgoid. j: optic Dwre. «*. »lis{ib«aoiil.
q. qoftdiatc^ j(L stepes- jr. lansti* Totandk. Am. hixiiMk-
tal semiciicukr muU. /at poateiior v«rtia«l aMUiiiuv-uW
caDAl : botb tlie anteno- kod the poaterior aNatoitv'ttUr
canals are sc«it i<h'"'"g through the CMtUl^t«. w. •ll|im>
occipital, ea. exoccipiUL oc oocipital ooudjrlo. «o. IHito-
chord. nuL Ueckels CMiilaga. «A. Mnto-hfkL U, bwi-
h;aL c6r. and <6r. OBrato-bnnchial Ur, bMibrauiihi«l.

part is known as the XaixmlL stAmoid rQ([imi, wUioh
is always perforated for the passagu of thu olfw^tory
nerve.

The sense capsules. The most iiupurtuit of thow

is the auditory capsule, which, oir wo huvu huuii, I\iiim

intimately with the lateral woIIh of ihu iikuU. in l^iint

there is usually a cleft separating it iVoii) Uiu tillii]i)lti

F. A B. V^

S42 ' THE FIt-TH DAY. [cHAP.

noid region of the skull, through which the third'
division of the fifth nerve passes out. This cleft be-
comes narrowed to a small foramen. The sclerotic is
free, but profoundly modifies the rL'n;ion of the cranium
near which it is placed. The na.sal investment is de-
veloped in continuity, and is closely united, with the
ethmoid region.

The cartila^nous cranium, the development of
which has been thus briefly traced, persists in the
adult without even the addition of membrane bones
in certain fishes, e.g. the £I^smobranchii. In the Sela-
chioid Ganoids it is also found in the adult, but is
covered over by membrane bones. In all other types
it ia invariably present in the embryo, but becomes in
the adult more or loss replaced by osseous tissue.

The bones in the adult skull may be divided
roughly into two categories according to their origin.

(1) Cartilage bones, i.e. ossifications in the primi-
tive cartilaginous cranium.

(2) Meuibrane bones, i.e. ossifications in membrane
without any cartilaginous precursors.

The names which have been given to the various
parts of the cartilaginous cranium in the above account
are derived from the names given to the bones appear-
ing in the respective regions in the more developed
skull

The skeleton of the vipceral arches. The visceml
arches were all originally branchial in function. They
supported tlie walla between successive branchial clefts.

ITie first arch (mandibular) has in all living forma
lost its branchial function, and its bar has become con-
certed into a supporting skeleton for the jaws.

rt ia known ai the UOeral tikatoid re^aa, iriiidi
ilw&n prorated tat tfae pueage of the ol&ctosj

^ Bense c^mleL The most important of Ui«8e
Mie aoditorj capsule, which, as we have seen, fdaea
iy with the latenl walls of th« skulL In front
t is osaall; a cleft separating it from the alisphe-

r.AB. \6

h«8e I

naea |

ixmt I

phe- I

244

THE FIFTH DAY.

[chap.

Meckel's cartilage (Fig. 79, mk.) ; it soon becomea
covered by investing (membrane) bones whicli form
the mandible; and its prdximal end ossifies as the
artieidare.

FiQ. 79.

ViBW FEOU BEi/OW OF THS Paired AppEyDAGES of the Sedll
Of A Fowl om the Fifth Day of Incdbation. (From
Parker.)

ov. I. cerebral veaidBs. «. eye. Jh. fronto-naoal process, n. nasal
pit. Ir. trabeoulcB. jHt. pituitary space, mr. superior
maxillary process, pff. pterygoid, pa. palatine, q. quad-
rate, mi. Meckera cartilage, ek, cerato-byal. bh. basi-
hyal. cbr. ceratobrancliiQi. ebr. proiiiiml portion of the
cartilage in the third viaceral arcli, bbr. boeibranchiaL
I. first visceral uli:ft. 2. second visceral cleft. 3. third vis-
ceral arch.

In the next arch, usually called the second visceral
or hyoid arch, there is a very small development of
cartiJage. This consists of a central azygos piece, ^e

TBS COLVUELLi}

'tasi-hyal' (Fig. 79, bk.). and two roda, o
^ife, tie 'cereto-hyals' (Fig. 79, ch.).

In the third arch, which corresponds with the i ___
wranchial arch of the Ichthyopsida, there is on each
"d© a lar^je diatal cartilaginous rod (Fig. 79, <ir.). the
•^fa to-branchial,' and a smaller proximal piece (Kg.
'"> *6r.); between the two arches liea an undefined
™*»s (Fig. 79. bbr), the ' basibranchiaL' In the arches
"^hind this one there is in the hird no development of
«wtaUge.

The lower part of the hjoid arch, including the
'***i-liyal, unites with the remnants of the arch behind
^ form the hyoid bone of the adult.

The fenestra ovalis and fenestra rotunda appear
■^ the seventh day as spaces in the side walb of the
P^'^otic cartilage. The former is filled up by a amall
pi*ce of cartilage, the stapes (Fig. 7S, tt.), which in the
^'^olt forms part of the columella (see pp. 166, 167).

The columella is believed by Huiley and Parker to represent
™ indepeodetitl J developed doraaJ elemeat of the hyoid, together
*^tii tbe atapee with which it has become united.

Sor further details of the development of the skull
*0 taust refer the student to Professor Parker's Memoir
"P^iii the Development of the Skull of the Common

^^^^l (Gallus domesticus), Phil. Trans., 1866, Vol CLVL,
T^ 1, and to the chapter on tbe Bird's skull in the
^orpholoff^ of the Skull, by Professor Parker and
*L«- Bettany.

We shall conclude this account by giving a table of
"lose bones which are preformed in cartilage, and of the
V^tely splint or membrane bones.

S46 THE FIFTH DAY. [cHAP.

jPorto of the bird^a skull which are either preformed
in cartilage or remain eartilaginojia.

Formed from the parachordal cartilages and their
upgrowths around the foramen magnum. — Supraocci-
pitaL Exoccipital. Basioccipital.

Formed in the periotic cartilage. — Epiotic. Prootic.
Opisthotic.

Formed from the trabeculse and their upgrowths.
— Alisphenoid. Basiaphenoid. Orbitosphenoid, Pre-
sphenoid. Ethmoid. Septum nasi, turbinals, prenaaal
and nasal cartilages.

Articulare and quadrate belonging to the first
visceral arch. Skeleton of the second and third visceral
arches and stapes.

Splint-bones not prefoniied in cartilage.

Parietals. Squamosals. Frontals. Lacrymala.
Nasals. Premaxillas. Maxillie. Masillo-patatinea.
Vomer. Jugals. Quadrato-jugals. Deutary and
bones of mandible. Basi- temporal and rostrum. Ptery-
goid and palatine (superior maxillary process).

The fece. Closely connected with the development
of the skull is the formation of the parts of the face.

After the appearance of the nasal grooves on the
fourth day tht mouth (Fig. 80, M.) appears as a deep
depression inclosed by five processes. Its lower border
ifi entirely formed by the two inferior maxillary pro-
cesaea (Fig. 80, f. 1), at its sides lie the two superior
maxillary processes iS. M., while above it is bounded by
the fronto-nasBX process nf.

Head op Embbto Chick of the Fourth Day tiewbd from
■*Low AS AN opAqDE OBJECT. (Clironiic acid preparation.)

1- cerebral hemiapheres. FB. vesicle of the third ventricle.
Op. eyeball, n/. oaao-fnmtal proceaa. M. cavity of mouth.
S.M. superior maiillary proceaa of F. 1, the first visceral
iold (mandibular arch). F. % F. 3, second and third
vJBoeral folds. N. nusa! pit.
to order to gain the view liere given the neck was cut acrow
*eeD the third and fourth viaceral folds. In the nectiou e
'"^ made, are seea the alimentarf canal al with its collapaed
^'Js, the neural canal m.c, the notochord ch., the dorsal aorta
1 and the jugular veina V.

fLl

Jifter a while the outer angles of the fronto-nasal
kclosing the expanded termination of the
^^Deculoe, project somewhat outwards on each side,
^*ijig the end of the process a rather bilobed appear-
"**i«. These projecting portions of the fronto-naaat pro-
j^B form on each side the inner margina of the rapidlj

248 THE FIFTH DAT. [CHiP.

deepening nasal grooves, and are sometimes spokeD uf
B8 the iniier nasal processes. The outer margin of aicli
nasal groove is raised up into a projection freqiieutlj
spoken of as tlie outej' nasal process, which runs down-
wards to join tke superior maxillary process, from whicb
however it is separated by a shallow depression. This
depression, which runs nearly horizon t-ally outwarfs
towards the eyeball, is known as the lacrj-mal gioore
(see p. 155).

On the fifth day the inner nasal processes, or lowa^
and outer comers of the fronto-nasal process, arching
over, unite on each side with the superior manlUry
processes. (Compare Fig. 81, which, however, is a ri
of the head of a chick of the sixth day.) In this vm^
each nasal groove is converted into a canal, whicb Itt*^

Ukad of a Chick at thb Sixth Day frok below. (Fr*""
Huilej.)

Ja. Mrebral veaiclas, u. eye, in which the reinaiaa of the ohort'''
Blit can etill be seen. g. nasal pita. i. fronto-nasal pM^'*'
I. BUpiirior maiiUary prooeaa. 1. inferior maiilkiy pro***

Vlll] THE MOUTH.

The cavity of the moxitli is seen encloaad by the fronto-naBal
process, the superior maxillary processes and the first pair of
risceral arches. At the bock of it is seen the opening leading
into the throat. The nasal grooves leading from the nasal pita
to the mouth are already closed over and converted into canals.

from the nasal pit above, into the cavity of the mouth
below, and places the two in direct communication.
This canal, whose lining consists of epiblaat, is the
rudiment of the nasal labyrinth.

By the seventh day (Fig. 82), not only is the union
of the superior maxillary and fronto-naaal processes
completed, and the upper boundary of the mouth thus
definitely constituted, but these parts begin to grow
rapidly forward, thus deepening the mouth and giving
rise to the appearance of a nose or beak (Fig, 82),
which, though yet blunt, is still distinct. The whole of
the lower boundary of the buccal cavity is formed by
the inferior maxillary processes.

As we have before mentioned (p. 240), cartilage ano-
ceeded by bone is developed in the fronto-nasal process ;
the pterygo-palatine osseous bar (membranous ossifica-
tion) in the superior maxillary process; Meckel's cartila^
the main part of which atrophica, the proximal end only
ossifying as the articulare, and the quadrate succeeded
by bone in the inferior maxillary process; the other
bones which form the boimdaries of the mouth in the
adult are developed later after all external trace of these
parts as separate processes has disappeared.

At first the mouth is a simple cavity into which the
nasal canals open directly. When bowev'st ^W -s^jcwkis.

THE FIFTH DAY. [CHAP.

Head of a Chick o^' the Seventh Da? tkom below. (FroTii

Huxlay.) ■
1(1. cerebnil vesicles, a. eje. g. uasal pite. t. fironto-nasal
prix:eas. I. superior maiUIary process. 1. first visoaral
arch. 2. Bewnd visceral arch. .!■. firat visceral cleft.
The external opening of the mouth has beuoma much cou-
stricted, but it ia still enclosed hy the fronto-nosal pi'ucess and
Buperiur maxillary processes above, and hy the infi^rior maiillary
processes (first pair of viscera! arches) below.

The superior maxillary processes have united with the ^nto-
nasal process, along the whole length of the latter, with the
exception uf a small space in &aat, where a narrow angular
opening is left between the two.

processes unite together to form the upper boundary of
the mouth, each superior maxillary process sends in-
wards a lateral bud. These buds become flattened and
form borizoutal plates which stretch more and more
inward towards the middle line. There they finally
meet, and by their union, which is effected first in &ont
aad thence extends backwards, constitute a horizontal

THE SPINAL COBD.

251

plate stretching right across the mouth and dividing
it into two cavities— an upper and a lower one.

In the front part of the mouth their union ia quite
complete.so that here there is no communication between
the two cavities. Behind, however, the partition is not
a complete one, so that the two divisions of the buccal
cavity communicate at the hack of the mouth. The
external opening of the mouth passes into the lower of
these two cavities, which may therefore be called the
mouth proper. Into the upper chamber the nasal
ducts open ; it may be called the respiratory chamber,
and forms the commencement of the chamber of the
noae. In birds generally the upper nasal cavity be-
comes aubsequentiy divided by a median partition into
two chambers, which communicate with the back of
the mouth by separate apertures, the posterior nares.
The original openings of the nasal pits remain as the
nostrils.

The spinal cord.— On this day important changes
take place in the spinal cord; and a brief history of
the development of this organ may fitly be introduced
here.

At the beginning of the third day the cavity of the
neural canal is still of considerable width, and when
examined in vertical section its sidi-s may he seen to be
nearly parallel, though perhaps approximating to each
other more below than above.

The exact shape vaiies according to the region of
the body from which the section is taken.

The epiblast walls are at this time composed of
radiately arranged columnar cells. The cells arc much
elongated, but somewhat irregular; and it \& ■s'sc^

25S THE FIFTH DAY. [CHAP.

difficult in sections to make out their individual
boundaries. They contain granular oval nuclei in
which a nucleolus can almost always be seen. The
walla of the canal are both anteriorly and posteriorly
considerably thinner in the median plane than in the
middle.

Towards the end of the third day changes take
place in the shape of the cavity. In the lumbar region
its vertical section becomes more elongated, and at the
same time very narrow in the middle while expanded
at each end into a somewhat bulbous enlargement, pro-
ducing an hour-glass appearance (Fig. 65). Its walls
however still preserve the same histological characteffi
as before.

On the fourth day (Fig. 68) coincidently with the
appearance of the spinal nerves, important changes
may be observed in the hitherto undifferentiated epi-
blastic walla, which result in its differentiation into (1)
the epithelium of the central canal, (2) the grey matter
of the cord, and (3) the external coating of white
matter.

The white matter is apparently the result of a
differentiation of the outermost parts of the superficial ,
cells of the cord into longitudinal nerve-fibres, which
remain for a long period without a. medullary sheath.
These fibres appear in transverse sections as small dota.
The white matter forms a transparent investment of
the grey matter; it arises as four patches, viz. an anterior
and a posterior white column on each aide, which lie on
a level with the origin of the anterior and posterior
nerve-roots. It is always, at first, a layer of extreme
teziuit/, but rapidly increasea in thickness in the sub-

VULj THE GREY MATTER.

tequeot stages, and extends ao as gradually to cover the
whole cord (Fig. 83).

SktiOH THBOliOB THE SPIKAL CoRD OF A SEVEN DaKs"

Chick.
^ donal vhite column, lew. lateral white column, atne. vou-
tral white coliumt. e. doraal tissue filling up the part where
tfae doraal baaiire will be formed, pe, dormil grej cornu.
dc anterior grey comiL ep. epithehal cells, age, anterior
wmiuiBaure. j^. dotsal part of spinal canaL tpc. ventral
part of spinal canaL a/, anterior fiasure.

The grey matter and the central epithelium are
famed by a differentiation of the main mass of the
*sll» of the medullary canal. The outer cells lose their

254 THK FIFTH DAY. [CHAI',

epithelial -like arraugement, and, becoming prolonged
into fibres, give rise to the grey matter, while the inner-
most cells retain thoir primitive arrangement, and con-
stitute the epithelium of the canal. The process of
formation of the grey matter would appear to proceed
from without inwards, so that some of the cells which
have, on the formation of the grey matter, an epithelial-
tike arrangement, subsequently become converted into
true nerve-cells.

The central epithelium of the nervous system pro-
bably corresponds with the so-called epidermic layer of
the epiblast.

The grey matter soon becomes prolonged dorsally
and ventrally into the posterior and anterior horns. Its
fibres may especially be traced in two directions : — (1)
round the anterior end of the spinal canal, immediately
outside its epithelium and so to the grey matter on
the opposite side, forming in this way an anterior grey
oommiasure, through which a decussation of the fibres
from the opposite sides is efi'ected: (2) dorsalwards
along the outside of the lateral walls of the canal.

There is at this period (fourth day) no trace of the
ventral or dorsal fissure, and the shape of the central
canal is not very different from what it was at an earlier
period. This condition of the spinal cord is especially
instructive as it is very nearly that which is permanent
in Auiphioxus.

The next event of importance is the formation of
the ventral or anterior fissure. This begins on the fifth
day and owes its origin to a downgrowth of the an-
terior horns of the coid on each side of the middle Una
The two downgrowths enclose between them a some-

vin.) THE posTEEiOR nastJRE. 255

what linear space — the anterior dssure — wMch in-
creases in deptli in the succeeding st^es (Fig. 83. af).

The dorsal or posterior tissure is formed at a later
peiiod (about the seventh day) than the anterior, and
^companies the atrophy of the dorsal section of the
embryonic^ly large canal of the spinal cord. The exact
nioJe of its formation appears to be still involved in
some obscurity.

It Mms probable, though further inTestigations am the pcnnt
»re rtDJ required, that the dorsal fissure is a direct result of the
•tftphj of the dorsal part of the central canal of the spinal
^ The ira]lB of thie ooalesoe dorsal!;, and the coaleaceooe
I'^'iiullj extends inwards, so aa finollj to reduce the central
wal to a Dunut« tube, formed of the rentraJ part of the original
•Wi*!. The epithehal wall formed bj the coalesced walk on the
''"'Ml nde of the cotuil ia gradually absorbed.

The epithelium of the central canal, at the period when ita
""phj cianmeneea, ia not covered dorsally cither by grey or
■hit* matter, so that, with the gradual reduction of the dorsal
P^rtnftJie canal and the absorption of the epithelial wall formed
'T the Fnaion of its two sides, a flasure between the two halves of
lie tpinal cord be^ximes formed. This fissure is the posterior or
'«>il fiBBore. Id the prooesa of its formation the white matter
' ■ tte dorsal homa becomes prolonged so ae to line ita walls ; and
■BrtJj after its formation the dorsal grey commissure makes ita
tpMnnce ; this is not improbably derived from part of the
litUium of the original central canaL

Meanwhile an alteration is taking place in the ex-
'*nial outline of the cord. From being, as on the
'''Unh and fifth days, oval in section, it becomes, chiefly
'Wdgh the increase of the white matter, much more
""^ly circuUx.

By the end of the seventh day the following im-

256 THE FIFTH DAY. [CHAP,

portant parts of the cord have been definitely es-
tablished :

(1) The anterior and posterior fissures.

(2) The anterior and posterior homa of grey
matter.

(3) The anterior, posterior and lateral columns
of white matter.

(4) The spinal canal.

As yet, however, the grey masses of the two aides of
the cord only communicate by the anterior grey com-
missure, and the white columns of opp()8ite sides do
not communicate at all. The grey matter, moreover,
still far preponderates over the white matter in
quantity.

By the ninth day the posterior fissure is fully
formed, and the posterior grey commissure has abo
appeared.

In the centre of the sacral enlargement this com-
missure is absent, and the posterior columns at a later
period separate widely and form the ' sinus rhomboi-
dalis,' which is not, as has been sometimes stated, the
remains of the primitive 'sinus rhomboidalia ' visible
during the second day.

The anterior white columns have much increased on
this day, and now form the sides of the already deep
anterior fissure. The antj?rior white commissure does
not however appear till somewhat later.

The Iieart. The fifth day may perhaps be taken
as marking a most important epoch in the history of
the heart. The changes which take place on that and
on the sixth day, added to those previously undergone.

THE VENTEICULAB SEFrOM.

-s of >^^^H
art ^^H

.in lying 1

■nsform the simple tube of the early days
nbatioQ into an almost completely formed heart.
The venous end of the heart, though still lying
oevhat to the left and dorsal, is now placed as far
brwanls as the arterial end, the whole organ appearing
to be drawn together. The ventricular septum is com-
ideU.

The apez of the ventricles becomes more and more
pointed In the auricular portion a smalt longitudinal
Slid appears as the rudiment of the auricular septum,
thile in the caJialia auricularis, which is now at its
pwi«st length, there ia also to be aeeu a commencing
tanaveise partition tending to separate the cavity of"
Die auricles from those of the ventricles.

About the 106th hour, a septum begins to make its
'ppearance in the bulbus arteriosus in the term of a
iwigitudinal fold, which according to Tonge (Proc.
^Sm/al Soc. 186S) starts, not (as Von Baer thought)
tthe end of the bulbua nearest to, but at that farthest
ftttoved fmrn, the heart. It takes origin from the wall
w the bulbus between the fifth and fourth paira of
A'ches and grows backwards in such a manner as to
4vide Uie bulbus into two channels, one of which leads
"Wo the heart to the fourth and third pair of arches
'XA the other to the fifth pair. The free edge of the
*ptiini is somewhat V-shaped, so that its two legs as
* *ere project backwards towards the heart, further
'^ its central portion ; and this shape of the free
Wge ia maintained during the whole period of its
Po»th, Its course backwards is not straight but
^1*1, and thus the two channels into which it divides
bulbus arteriosus wind spirally the one over the
P. 4 B. \1

258 THE FIFTH DAY. [CHAP.

other. The existence of the septum can only be as-
certained at this stage by dissection or by sections,
there being as yet no external signs of the division.

At the time when the septum is first formed, the
opening of the bulbus arteriosus into the ventricles ia
narrow or alit-like, apparently in order to prevent the
flow of the blood back into the heart. Soon after the
appearance of the septum, however, semilunar valvea
(Tonge, loc. cit.) are developed from the wall of that
portion of the bulbus which lies between the free edge
of the septum and the cavity of the ventricles.

These arise as six solid outgrowths of the wall
arranged in pairs, a ventral, a dorsal, and an outer pair,
one valve of each pair belonging to the one and the
other to the nther of the two main divisions of the
bulbus which are now being established.

The ventral and the dorsal pairs of valves axe the
first to appear: the former as two small prominences
separated from each other by a narrow groove, the
latter as a single shallow ridge, in the centre of which
is a prominence indicating the point where the ridge
will subsequently become divided into two. The outer
pair of valves appear opposite each other, at a con-
siderably later period, between the ends of the other
pair of valves on each side.

As the septum grows backwards towards the heart,
it finally reaches the position of these valves. One of
its legs then passes between the two ventral valves,
and the other unites with the prominence on the dorsal
valve-ridge. At the same time the growth of all tie
parts causes the valves to appear to approach the heart
and thus to be placed quite at the top of the ventriculai-

Tin.] THE BULBU3 ARTERIOSUS. 269

cavities. The free edge of the septum of the bulbus

now fuses with the ventricular septum, and thus the
division of the bulbus into two separate channels, each
provided with three valves, and each communicating
with a separate side of the heart, is complete, the po-
sition of the valves not being very different from what
it is in the adult heart

Tliat division of the bulbus which opens into the
fifth pair of arches is the one which communicates with
the right ventricle, while that which opens into
the third and fourth pairs communicates with the left
ventricle (vide Fig. 93). The former becomes the pul-
monary artery, the latter the commencement of the
systemic aorta.

The external constriction actually dividing the bul-
bus int« two vessels does not begin to appear till the
septum has extended some way back towards the heart.

The semihmar valves become pocketed at a period
considerably later than their first formation (from the
147th to the 165th hour) in the order of their ap-
pearanee.

Towards the end of the fifth and ia the course of the
sixth day further important changes take place in the
heart.

The venous end with its two very conspicuous au-
ricular appendages, comes to be situated more dorsal
to the arterial end, though it still turns rather towards '
the left. The venous portion of the heart undei^oes
on the sixth day, or even near to the end of the fifth,
such a development of the muscular fibres of its walls
that the canalis auricularia becomes almost entirely
concealed. The point of the tewrt, is "ii'^'w &iiacN*5i.
\1— t

860

THE FIFTH DAY.

[chap.

nearly backwards (i.e. towards the tail), but also a little
ventral ward 3.

An alteration takes place during the sixth day in
the relative position of the parts of the ventricular
division of the heart. The right veutricle is now turned
towards the abdominal surface, and also winds to a
certain extent round the left ventricle. It will be
remembered that on the fourth day the right ventricle
was placed dorsal to the left.

The right ventricle is now also the smaller of the
two, and the constriction which divides it from the left
ventricle does not extend to the apex of the heart
(Fig. 84), It has, however, a very marked bulge to-
wards the right.

Fio. U.

Two VraWB OF THE Heabt of a Chick upon the Futh

Dat oj" iKcnflATioN.

A. from the ventraJ, B. from the doraal side.

t.a. left Buricul^ir appendage, r.o. right auricular appendage.

r.v. right ventricle, l.p. lefb veatricle. b. bulbus arteriosus.

At first the bulbus arteriosus appeared to come off
chiefly from the left ventricle ; during the fifth day, and
Btill more on the sixth, it appears to come from the

niL]

THE BCLBUS AKTERIOSDS,

261

Kglit chamber. This is caused by the canal from tbe
right ventricle into the bulbus arteriosus passing to-
wards the left, and on the ventral aide, so as entirely
to wmceal the origin of the canal from the left chamber
ot the heart. On the seventh day the bulbus artenosuB

^yeara to come less markedly from the right ade of

Hfte heart.

H M these changes, however, of position of the bulbua

^luteriosua only affect it externally ; during the whole
time the two chambers of the heart open respectively
into the two divisions of the bulbus arteriosus. The
iwelling of the bulbua la much less marked on the
erentb day than it was before.
I At the end of the sixth day, and even on the fifth
|d»y (Figs. 8*. 85), the appearance of the heart itself.

y^lv

Hkakx or A Cbiok dpok the Siitb Day op Inccbatios,
raoH THR VsNTaAL Subface.
. left auriculax appendiige. r.a. rigM auriculai' appeodage.
r.*. ri^t ventricle, l.v. left ventricle, b. bulbus arteriosus.

at reference to the vessels which come from it,
■ not very disajmilor from that which it presents when

rhen I

I
I

262 THE FIFTH DAY. [CHAP,

The original curvature to the right now forms the
apex of the ventricles, and the two auricular appendages
are placed at the anterior extremity of the heart.

The most noticeable difference (in the ventral view)
is the still externally undivided condition of the bulbiis
arteriosus.

The subsequent changes which the heart undergoes
are concerned more with its internal structure than
with its external shape. Indeed, during the next three
days, viz. the eighth, ninth, and tenth, the external
form of the heart remains nearly unaltered.

In the auricular portion, however, the septum which
commenced on the fifth day becomes now more con-
spicuous. It is placed vertically, and arises from the
Tentral wall ; commencing at the canalis auricularis
and proceeding backwards, it does not as yet reach the
opening into the emus venosus.

The blood from the sinus, or, as we may call it, the
inferior vena cava, enters the heart obliquely from the
right, 90 that it has a tendency to flow towards the loft
auricle of the heart, which is at this time the larger of
the two.

The valves between the ventricles and auricles are
now well developed, and it is about this time that the
division of the bulbus arteriosus into the aorta and
pulmonary artery becomes visible on the exterior.

By the eleventh or thirteenth day the right auricle
has become as large as the left, and the auricular sep-
tum much more complete, though there is still a small
opening, the foramen ovale, by which the two cavities
communicate with each other. Through this foramen
the greater part of the blood of the vena cava inferior.

via] THE EUSTACHIAN VALVE. 263

"bicli is DOW joined just at ita entrance Into the heart
bj the right vena cava superior, is directed into the left
luncle. The left vena cava superior enters the right
siiricle independently; between it and the inferior vena
cava ia a small valve which directs ita blood entirely
into the right auricle.

On the sixteenth day the right vena cava superior,
«hen viewed from the exterior, still appears to join the
tnferitjT vena cava before entering the heart ; from the
interior however the two can now be seen to be sepa-
rated by a valve. This valve, called the ' Eustachian
™'ve,' extends to the opening of the left vena cava
snperior, and into it the valve which in the earlier
''•ge separated the left superior and inferior venae
ttv* liaa apparently become merged. There is also on
'k left side of the opening of the inferior cava a mem-
°nDe stretching over the foramen ovale, and serving as
* '^ve for that orifice. The blood from the inferior
*'» still passes chiefly into the left auriclo through
tit foramen ovale, while the blood from the other
'•TO venae cava; now falls into the right auricle, being
Pfpvetiied from entering the left chamber by the
'■•Ktachian valve.

Htnce, since at this period also the blood from the
''ft ventricle passes to a great extent to the anterior
portion of the body, there is a species of double-circula-
lion going on. The greater part of the blood from the
'^tois entering the left auricle from the inferior vena
ta^a passes into the left ventricle and is thence sent
•^hiygjy, t|,g head and anterior extremities through the
™inl and fourth arches ; from these it is brought back
lirough the right auricle to the right ventricle, froqi

864 THE FIFTH DAT. [CHAP,

wlieDce through the fifth arch it is returned along the
aorta to the allantois.

From the Bcventeenth to the nineteenth day the
right auricle becomes lai^er than the left The large
Eustachian valve still prevents the blood from the
superior cavjB from entering the left auricle, while it
conducts the blood from the inferior vena cava into that
chamber through the foramen ovale. The entrance of
the inferior vena cava is however further removed than
it was from the foramen ovale, and the increased flow
of blood from the lungs prevents all the blood of the
inferior cava from entering into the left auricle. At
the same time the valve of the foramen ovale prevents
the blood in the left auricle from entering the right
auricle.

During the period from the seventh day onwards
the apex of the heart becomes more marked, the arte-
rial roots are more entirely separated and the various
septa completed, so that when the foramen ovale is
closed and the blood of the inferior vena cava thereby
entirely contined to the right auricle, the heart has
practically acquired its adult condition.

The pericardial and pleural cavities. The heart
at first lies in the general body cavity attached to the
ventral wall of the gut by a meaocardium (Fig. 86, A),
but the part of the body cavity containing it afterwards
becomes separated off as a distinct cavity known as the
pericardial cavity. It is formed in the following way.
When the two ductus Cuvieri leading transversely from
the sinus venosus to the cardinal veins become deve-
loped (p. 170), a horizontal septum is formed to support
them, stretching across from the splanchnic to the ao-

THE PERICARDIAL CAVITY.

matic aide of the body cavity, dividing the body cavity
for a short distance in this region into a dorsal Bection,
(formed of a right and a left division) constitutiog the
true body cavity (Fig. 86 B, p.p), and a ventral section
(Fig. 86, B, p.c), the pericardial cavity. The two parts
of the body cavity thua formed are at first in free com-
muDication both in front of and behind this septum. The

TRAHSvitasB ^ecTioHa thkdcoh a CaiCE EitBRYo with
TwENTY-osB Mbsoblastic SoMiTEa to baew the foriu-

IION Of THE pKRICABDIAt CaVITT, A. BEING THE AUTB-

RiOB Section.
pj). bodj cavity, pe. pericardial cavity, al, alimentary cavity,
auricle. V. ventricle, iv. sinus veoosus. de. ductus
Cuvieri. oo. aorta. «ip. muBCle-plate. nu. madullBC^ vnci.

kpp. body .
Cuvii

septum however ia aoou continued forwards so as oom-
pletely to separate tlie ventral pericardial and lie
dorsal bodi/ cavity in front, the ^pericardial cavitj ai-
tending considerably further forwards than Uie bod)'
cavity.

Fio. 87.

Seoiioh thbocoh thk Cahdiac Kuuiou of as Eubbto or
Lacbrta. Muralis of 9 m.h. to bhew thi modi C
formation or the Peeicabdial Cavity.

ht, heart, pe. pericardial cavity, al. alimeutui7 tract Ig. batit-
I. liver, pp. body cavity, md. open end of MiiUeriao du*
tad. Wolffian duct. tie. vena cava inferior, ao. aorta- *
notochord. me. medullnry cord.

Since the horizontal septum, by ita mode of origiff
is necessarily attached to the ventral side of the gnt^
the dorsal part of the primitive body space b, aa *"
have aJready mentioned, divided into two halree bf
s median vertical septum formed of the gut aod its

THE PEBiCAHDUL CAVITY.

mesentery (Fig. 86, B). Posteriorly the horizontal sep-
tum grows in a slightly ventral direction along the
under surface of the liver (Fig. 87), till it meets the
abdominal wall of the body at the insertion of the
falcifonn ligament, and thus completely shuts off the
pericardial cavity from the body cavity. The horizontal
septum forms, as is obvious from the above description,
the dorsal wall of the pericardial cavity.

After the completion of this separation the right
and left sections of the body cavity, dorsal to the peri-
cardial cavity, rapidly become larger and receive the
lungs which soon sprout out from the throat.

The diverticula which form the lungs grow out into
splanchnic mesoblast, in front of the body cavity, but
as they grow they extend into the two anterior com-
partments of the body cavity, each attached by its
mesentery to the mesentery of the gut (Fig. 87, Ig,).
They soon moreover extend beyond the posterior limit of
the pericardium into the undivided body cavity behind.

To understand the further changes in the peri-
cardial cavity it is necessary to bear in mind its rela-
tions to the adjoining parts. It lies at this period
completely ventral to the two anterior prolongations of
the body cavity containing the lungs. Its dorsal wall is
attached to the gut, and is continuous ^vith the me-
sentery of the gut passing to the dorsal abdominal wall,
forming the posterior mediastinum of human anatomy.

The changes which next ensue consist essentially in
the enlargement of the sections of the body cavity
dorsal to the pericardial cavity. This enlargement
takes place partly by the elongation of the posterior
mediastinum, but still more by the t<NQ di\'^s^civfi> ^ 'QeiSi

268 THE FIFTH DAY. [C2iP.

body cavity which contain the lungs extending ttem-
Belvea ventrally round the outside of the pericaidial
cavity, Tliis process ia illustrated by Fig. 88, UVeii

Section

ROnSDED

Advanced Embrto oi
THE Pericardial Cavity
THE Pleural Cavities.

hi. heart pc. jiorioordial cavity, pl.p. pleural cavity. Ig.
al. alimentary tract, ao. dorsal aorta, cA. cotoohml. V-
rib. 6l. stemuin. sp.i:. spinal cord.

from an emhryo rabbit. The two dorsal sections of th«
body cavity (pl.p.) finally extend so as completely
envelope the pericardial cavity (pc), remaining ho*'
ever separated from each other below by a lamina eX'
tending from the ventral wall of the pericardial ca"^

Vm.] mSTOLOQlCAL DIFFERENTIATION. 269

to the body wall, which forms the anterior mediastinum
of biunan anatomy.

By these changes the pericardial cavity ia converted
into a closed hag, completely surrounded at its aidea by
the two lateral halves of the body cavity, which were
primitively placed doraally to it. These two sections of
the body cavity, which in the chick remain in free
communication with the undivided peritoneal cavity
behind, may. from the fact of their containing the
lungs, be called the pleural cavities.

Hifitological differeutiation. The fifth day may also
be tak^n as marking tlie epoch at which histological
differentiation first becomes distinctly established and
begins to make great progress.

It is of course true that loug before this date, even
from the earliest hours, the cells in each of the three
fundamental layers have ceased to be everywhere alike.
Nevertheless the changes undergone by the several cells
have been few and slight. The cells of epiblastic origin,
both those going to form the epidermis and those in-
cluded in the neural involution, are up to this time
simple more or less columnar cells ; they may be seen
here elongated, there oval, and in another spot spheroi-
dal ; here closely packed, with scanty protoplasm, there
scattered, with each nucleus well surrounded by cell-
substance ; but wherever they are found they may still
be recognized as cells of a distinctly epithelial character.
So also with the cells of hypoblastic origin, whether
simply lining the alimentary canal or taking part in the
formation of the compound glands. Even in the meso-
blaat, which undergoes far more changes than either of
the other layers, not only increasing more rapidly «i

THE FIFTH DAT,

bult but also serving as the mother tissue for a fitf
greater number of organs, the alterations in the indi-
vidual cells' are, till near upon the fifth day, insigmfi-
cant. Up to this time the mesoblaat may be spoken of
as consisting for the most part of little more thra in-
different tissue : — of nuclei imbedded in a protoplasmic
call-substance. In one spot the nuclei are closely
packed together, and the cell-substance scanty and
compact; at another the nuclei are scattered abon*
with spindle-shaped masses of protoplasm attached l*>
each, and there is a large development either of inter-
cellular spaces or of intracellular vacuoles filled witto-
clear fluid. The protoplasm differs in various places^
chiefly in being more or less granular, and less
transparent, having as yet undergone but slight chemi-
cal transformation. Up to this epoch (with the excep-
tion of the early difiereutiated blood and muscles of tb.^--
muscle plates) there are no distinct tissues, and tba
rudiments of the various organs are simply marked ovt
by greater or less condensation of the simple meao-
blastic substance.

From the fifth day onwards, however, bietologick)
diflferentiation takes place rapidly, and it soon becomes
possible to speak of this or that part as being composed
of muscular, or cartilaginous, or connective, &c. tiBdift-
It is not within the scope of the present work to treat
in detail of these bistogenetic changes, for infonnatioo
concerning which we would refer the reader to hifltcl*-
gical treatises. We have already had occasion to refer

■ With the exception of tlie celk of the middle part of the iu^
layej of the muBclt-plBlea, tcliich we have Been became coaierK^ i"*"
longitn^al ntoBcleB on tlie third dny Ip. 187).

P^VHl] the epiblast. 271

incidentally to many of the earliest histological events,
and shall content ourselves by giving a brief summarj'
of the derivation of the tissues of the adult animal from
the three primary layers of the blastoderm.

The epiblast or upper layer of many emhryologists
forms primarily two very important parts of the body,
viz. the central nervous system and the epidermis.

It is from the involuted epiblast of the neural tube
that the whole of the grey and white matter of the
brain and spinal cord appears to be developed, the
simple columnar cells of the epiblast being apparently
directly transformed into the characteristic multipolar
nerve-cells. The whole of the sympathetic' nervous
system and the peripheral nervous elements of the
body, including both the spina! and cranial nerves and
ganglia, are epiblastic in origin.

The epithelium (ciliated in the young animal) lining
the canalis centralis of the spinal cord, together with
that lining the ventricles of the brain, all which cavities
and canals are, as we have seen, derivatives of the
primary neural canal, is the undifferentiated remnant of
the primitive epiblast.

The epiblast, as we have said, also forms the epider-
mis, not however the dermis, which is of mesoblastic
origin. The line of junction between the epiblast and
the mesoblaat coincides with that between the epidermis

' The details of the dSTelopment of the ejmpatlietio Bjatom h&ve
only been imparfeotl; worked out in the chiok. Wo propoeo defHRing
oar aaoonnt of what ia knou-n on tliix head to the second part at thia
work deaJing with the Mammalia. We ma; bete state, howovi^r, that the
whole of the chain of the sympathetic ganglia ia dereloped in eon-
tinnlt}' with the outgrowths from the wall of the neural tabs wluoh
give rise to the spinal neivea.

THE FIFTH DAY. [CHiT.

and the dermis. B'rom the epiblast are formed all sucb
tegumentary organs or parts of organs as are epideimic
in nature.

In addittoa to these, the epiblast plays an unporUaC
part in the formation of the organs of special sense.

According to their mode of formation these organ*
may be arranged into two divisions. In the first come
the cases where the sensory expansion of the orgaa of*
special sense is derived from the involuted epiblast i>f
the medullary canal. To this class belongs the
including the epithelial pigment of the choroid, whicli
is formed &om the original optic vesicle budded ou'fe
from the fore-brain.

To the second class belong the epithelial ej
of the membranous labyrinth of the ear and the canty
of the nose, which are formed by involution from the
superficial epiblast covering the external surface of the
embryo. Thase accordingly have no primary coonectioa
with the brain. We may also fairly suppose that the
'taste bulhs' and the nervous cells, which have lately
been described as present in the epidermis, are also
structures formed from the epiblast.

In addition to these we have the crystalline Ian*
formed of involuted epiblaat, and the cavity of tl»
mouth and anus lined by it. The pituitary bodj i*
also epiblastic in origin. These are the most important
parts which are derived from the epiblast.

From the hypoblast are derived the epithelium of
the digestive canal, the epithelium of the tracht*
bronchial tubes and air cells, the cylindrical epitheluim
of the ducts of the liver, pancreas and other gland* "•
the alimentary canal, as well as the hepatic cella cOD-

Vin.] THE HYPOBLAST AND MESOBLAST. 273

stituting the parenchyma of the liver, developed, as we
have Been, from the hypoblast cylinders given off around
the primary hepatic diverticula.

Homologous, probably with the hepatic cells, and
equjilly of hypoblastic origin, are tlie more spheroidal
' secreting cells ' of the pancreas and other glands. The
epithelium of the salivary glands, though these so exactly
resemble the pancreas, is of epiblastic origin, inasmuch
as the cavity of the mouth (p. 119) is entirely lined by
epiblast.

The hypoblast lines the allantois, and the notochord
also ia an hypoblastic product.

From the mesoblast axe formed all the remaining
parts of the body. The muscles, the bones, the connec-
tive tissue and the vessels, both arteries, veins, capillaries
and lymphatics, with their appropriate epithelium, are
entirely formed from the mesoblast

The generative and urinary organs are also de-
rived from the mesoblaat. It ia worthy of notice that
their epithelium, though resembling the hypoblastic
epithelium of the alimentary canal, is undoubtedly
mesoblastic.

From the mesoblast laatly are derived all the mus-
cular, connective and vascular elements, as well of the
alimentary canal and its appendages as of the skin and
the tegumentary organs. Just as it is only the epider-
mic moiety of the latter which is derived from the
epiblast, so it is only the epithelium of the fonner
which comes from the hyijoblast.

The important events then which characterize the
fifth day are ; —

1. The growth of the allantois.
F. ft B. ^S>

274

THE FIFTH DAY.

[chap. Vlll.

2. The appearaace of the knee and elhow^, and of
the cartilages which precede the bones of the digits and
limbs.

3. The formation of the primitive cartilaginous
cranium, more especially of the investing mass and the
trabeculffi, and the appearance of rods of cartilage in
the visceral arches.

i. The developments of the parts of the face : the
closing in of the nasal passages by the nasal processes.

5. A large development of grey matter in the
spinal cord as the anterior and posterior comua; con-
siderable gro\vtb both of the anterior and posterior
white columns, and the commencement of the anterior
and posterior fissures.

6. The appearance of the auricular septum, of a
septum in the bulbus arteriosus, and of the semilunar
valves.

7. The eatabUshment of the several tissues.

CHAPTER IX.

FBOM THE SIXTH DAY TO THE END OP INCUBATION.

The sixth day marks a new epoch in the develop-
ment of the chick, for distinctly avian characters then
first make their appearance.

Striking and numerous as are the features, which
render the class Aves one of the most easily recognizable
in the whole animal kingdom, the embryo of a bird does
not materially differ in its early phases from that of a
reptUe or a mammal, even in the points of structure
which are most distinctively avian. It may, it is true,
be possible to infer, even at a comparatively early stage,
from some subsidiary tokens, whether any given em-
bryo belongs to this class or that (and indeed the same
inference may be drawn from the ovum itself) ; but up
to a certain date it is impossible to point out, in the
embryo of the fowl, the presence of features which may
be taken as broadly characteristic of an avian organiza-
tion. This absence of any distinctive avian differen-
tiation lasts in the chick roughly speaking till the com-
mencement of the sixth day.

276

THE SIXTH DAY.

[chap.

We do not mean that uu the sixth day aJl the orgiuu
suddenly commence to exhibit pecuharitius which mark
them as avian. There are no strongly marked breaks
ill the history of development; its course is perfectly
gradual, and one stage passes continuously into the
next. The sixth and seventh days do however mark
the commencement of the period in which the spe-
cialization of the bird begins to be apparent. Then for
the first time there become visible the main features
of the characteristic manus and pes ; the crop and the
intestinal caeca make their appearance ; the stomach
takes on the form of a gizzard ; the nose begins to de-
velope into a beak ; and the commencing bones of the
skull arrange themselves after an avian type. Into
these details we do not propose to enter, and shall
therefore treat the history of the remaining days with
great brevity.

We will first speak of the f(etal appendages.

On the sixth and eeventh days these exhibit
changes which are hardly less important than the
events of previous days.

The amnion at its complete closure on the fourth
day very closely invested the body of the chick; the
true cavity of the amnion was at that time therefore very
small. On the fifth day fluid begins to collect in the
cavity, and raises the membrane of the amnion to some
distance from the embryo. The cavity becomes still
larger by the sixth day, and on the seventh day is of
very considerable dimensions, the fluid increasing with
it. On the sixth day Von Baer observed movements of
the embryo, chiefly of the Umbs; he attributes them
to the stimulation of the cold air on opening the egg.

IX.] THE YOLK. 277

By the seventh day very ohvious movements begin to
appear in the amnion itself; slow vermicular con-,
tractions creep rythmically over it. The amnion in
fact begins to pulsate slowly and rythmically, and by
its pulsation the embryo is rocked to and fro in the
egg. This pulsation is due probably to the contraction
of involuntary muscular fibres, which seem to be present
in the attenuated portion of the mesoblast, forming
part of the amniotic fold. (Cf. Chap. II. p. 46.) Similar
movements are also seen in the aUantois at a con-*
siderably later period.

The growth of the allantois has been very rapid,
and it forms a flattened bag, covering the right side of
the embryo and rapidly spreading out in all directions,
between the primitive folds of the amnion, that is be-
tween the amnion proper and the false amnion (serous
membrane). It is filled with fluid, so that in spite of
its flattened form its opposite walls are distinctly sepa-
rated firom each other.

The vascular area has become still further extended
than on the previous day, but with a corresponding loss
in the definite character of its blood-vessels. The sinus
terminalis has indeed by the end of the seventh day
lost all its previous distinctness, and the vessels which
brought back the blood from it to the heart are no
longer to be seen.

Both the vitelline arteries and veins now pass to
and frx)m the body of the chick as single trunks, as-
suming more and more the appearance of being merely
branches of the mesenteric vessels.

The yolk is still more fluid than on the previous
day, and its bulk has (according to Yon Baer") vcLCxoAa^.

S78

THE SIXTH DAY,

[chap.

This can only be due to its absorbing the white of the
ugg, which indeed is diminishing rapidly.

Daring the eighth, ninth, ajid tenth days the
amnion does not undergo any very important changes.
Its cavity is still filled with fluid, and on the eighth
day its pulsations are at their height, henceforward
diminishing in intensity.

The splitting of the meaoblast has now extended to
the outer Umit of the vascular area, viz. over about
three quarters of the yolk-sac. The somatopleure at
this point ia continuous (as can be easily seen by
reference to Fig. 9) with the original outer fold of
the amnion.

It thus comes about that the further splitting of the
mesoblast merely enlarges the cavity in which the
allantois lies. The growth of this organ keeps paoe
with that of the cavity in which it ia placed. Spread
out over the greater part of the yolk-sac as a flattened
bag filled with fluid, it now serves as the chief organ of
respiration.

Hence it is very vascular, the vessels on that side of
the bag which is turned to the serous membrane and
shell being especially large and numerous.

The yolk now begins to diminish rapidly in bulk.
The yolk-suc becomes flaccid, and on the eleventh day
is thrown into a series of internal folds, abundantly
supphed with blood-vessels. By this means the surface
of absorption is largely increased, and the yolk is more
and more rapidly taken up by the blood-vessels, and in
a partially assimilated condition transferred to the body
of the embiyo.

By the eleventh day the abdominal parietea though

H.] THE ALLANT0I3. 279

Btiil much looser and less finn than the walls of the
chest may be said to be definitely established, and the
loops of intestine, which have hitherto been hanging
down into the somatic stalk, are henceforward confined
mtliin the cavity of the abdomen. The body of the
embryo is therefore completed ; but it still remains
connected with its various appendages by a narrow
somatic umbilicus, in which run the stalk of the allan-
tois and the solid cord suspending the yolk-sac.

The cleavage of the mesoblast still progressing, the
yolk is completely invested by the (splanchnopleuric)
yolk-sae except at the pole opposite to the embryo,
where for some little time a small portion remains
unenclosed ; at tliis spot the diminished white of the
egg adheres as a dense viscid plug.

The allantois meanwhile spreads out rapidly, and
lies over the embryo close under the shell, being sepa-
rated from the shell membrane by nothing more than
an attenuated membrane, the serous membrane, formed
out of the outer primitive fold of the amnion and the
remains of the vitelline membrane. With this serous
membrane the allantois partially coalesces, and in
opening an egg at the later stages of incubation, unless
care be taken the allantois is in danger of being torn
in the removal of the shell membrane. As the allantois
increases in size and importance, the allantoic vessels
are correspondingly developetl. They are very con-
spicuous when the ^g is opened, the pulsations of the
allantoic arteries at once attracting attention.

On about the sizteeatli day, the white having

entirely disappeared, the cleavage of the mesoblast is
carried right over the pole of the yolk oijyswAB "OaKi

280' THE SIXTH DAY. [CHAP.

embryo, and is thus completed (Fig. 9). The yolk-sac
now, like the allantois wliich closely wraps it all round,
lies loose in a space bounded outside the body by the
aerouB membrane, and continuous with the pleuro-
peritoneal cavity of the body of the embryo. Deposits
of urates now become abundant in the allantoic fluid.

The lotise and flaccid walla of the abdomen enclose
a space which the empty intestines are far from filling,
and on the nineteenth day the yolk-sac, diminished
greatly in bulk but still of some considerable size, is
witiidrawn through the somatic stalk into the ab-
dominal cavity, which it largely distends. Outside the
embryo there remains nothing now but the highly
vascular allantois and the practically bloodless serous
membrane and amnion. The amnion, whose fluid during
the later days of incubation rapidly diminishes, is con-
tinuous at the umbilicus with the body-walls of the
embryo. The serous membrane (or outer primitive
amniotic fold) is by the completion of the cleavage of
the mesoblast and the invagination of the yolk-sac,
entirely separated from the erabiyo. The cavity of the
allantois by means of its stalk passing through the um-
bilicus is of course continuous with the cloaca.

In the EMBRYO itself a few general points only de-
serve notice.

B7 the Bixtli or seventh day the flexure of the
body has become leas marked, so that the head does
not lie 30 near to the tail as on the previous days ; at
the same time a more distinct neck makes its ap-
pearance.

Though the head is still disproportionately large, its
^owth ceases to be greater than that of the body.

IX.] THE BRAIN. 281

Up to this period the walls of the somatic stalk
have remained thin and flaccid, almost membranous in
fact, the heart appearing to hang loosely oat of the
body of the embryo. About this time however the
stalk, especially in front, rapidly narrows and its meso-
blast becomes thickened. In this way the heart and
the other thoracic viscera are enclosed by definite firm
chest walls, along the sides of which the ribs grow
forwards and in front of which the cartilaginous rudi-
ments of the sternum appear.

The abdominal walls are also being formed, but not
to the same extent, and the stalk of the allantois still
passes out from the peritoneal cavity between the
somatic and the splanchnic stalks.

In the brain one of the most marked features is the
growth of the cerebral hemispheres. The median division
between these has in front increased in depth, so that
the lateral ventricles are continued forwards as two
divergent horns, while backwards they are also con-
tinued as similar divergent horns separated from one
another by the vesicle of the third ventricle.

We propose to treat more fully of the development of the
brain in the second part of this work, the importance of the
mammalian brain rendering it undesirable to go too much into
the details of the brain of the bird.

All the visceral clefts are closed by the seventh day.
It will be remembered that the inner part of the first
cleft persists as the Eustachian tube (p. 166).

The structures which surround the mouth are be-
ginning to become avian in form, though the features
are as yet not very distinctly marked

iSS 'IH£ iilXTU DAY. [cHAP.

The tongue has appeared on the floor of the mouth
as a bud of mesoblast covered by epiblast.

During the eighth, ninth, and tenth days the
embryo grows very rapidly, the head being still especially
large, and at the same time becoming more round, the
mid-brain not being so prominent.

From the eleventh day onwards the embryo anc-
cessively puts on charactora which are not only
avian, but even distinctive of the genus, species and
variety.

So early as the ninth or tenth day the sacs con-
taioJQg the feathers begin to protrude Irom the sur&cs
of the skin as papillee, especially prominent at first along
the middle line of the back from the neck to the rump,
and over the thighs, the sacs of the tail feathers being
very conspicuous. On the thirteenth day these sacs,
generally distributed over the body, and acquiring the
length of a quarter of an inch or more, appear to the
naked eye as feathers, the thin walls of the sacs allow-
ing their contents, now coloured according to the variety
of the bird, to shine through. They are still however
closed sacs, and indeed remain such even on the nine-
teenth day, when many of them are an inch in length.

Feathers are epiJennol structoreB. The; arise from an in-
duration of the epidermis of papillie containing a vaaculu core

On the eighth day a chalky- looking patch is ob-
servable on the tip of the nose. This iDy the twelfth
day has become developed into a homy but still soft
beak.

On the thirteenth day, nails are visible at the ex-
tremities, and scales on the remaining portions of the

E sixteenth day become harder and
□ the beak,

special r^ona of tbs epidermis,

» noil bods. Tboj are formed b^ tbo

r of cells whiob makes itn appeurance

) lajers of the epidermis. The

i nail MOB becomes free, and the further

f additionB to the under aide and attached

1 day the carti]agmous skeleton is
ions muscles of the body can be
e clearness.

] according to Von Baer on the

y small deposits in the tibia, in

F the hind-limb, and in the sca-

mth or twelfth day a multitude of

. make their appearance in the

r and pelvic arches, in the ribs, in

md dorsal vertebra! and in

I, the centres of ossification of the

r found till the thirteenth day.

icb we have thus briefly narrated are

nportant changes in the arterial

r the venous system at about the

i fully described in Chap. vi.

s which have taken place between

r days of incubation may be seen

1 Fig. 58 with the diagrams

\a.j, nearly the whole of the venous
f of the embryo was carried back to

284 THE SIXTH DAY. [cHAP.

the heart by two main venous trunks, the superior (Fig.
58, J) and inferior (Fig. 58, 0) cardinal veins, joining
(in each side to form the short tranaverse ductus Cuvieri,
both which in turn united with the sinus venosus close
to the heart. Aa the head and neck continue to enlarge
and the wings become developed, the single superior
cardinal or jugular vein, as it is usually called (Figs. 89,
90, /), of each side, is joined by two new yeins : the

Di^sRAK OF Tai Vbhodb Cibculation at the Coimsvotiaan

Of THE Firrn Day.

H. heart d.e. ductus CuTiari. Into the ductus Cuvieri ofaaoli
aide fell J. the jugulw vi'in, W. the vein from the wing and
C. the inferior canliaal vein. S. V. ainufl venoaus. Of. vitel-
line vein. (7. allantoic vein, which at this stage gives off
branches to the bodf-walla. V.C.I, inferior vena cara.
I. liver.

vertebral vein {8u. J. V.), bringing back blood from the
head and neck, and the vein from the wing ( W).

The inferior cardinal veins have their roots in the
WolSan bodies; they become developed, pari passu.

' IX,] THE VENOUS SYSTEM. 285

with those organs, and may be caUed the veins of the
WolfBan bodies, On the third day they are the only
veins which bring the bloud back troui the hinder part
of the body of the embryo.

About the fourth or fifth day, however, a new single
venous trunk, the vena cava iiiferior (Fig. 89, V.GJ.),
makes its appearance in the middle line, in a plane more
dorsal than that of the cardinal veins, This, starting
from the sinua venosus not far from the heart, is on the
fifth day a short trunk running backward in the middle
line below the aorta, and speedily losing itself in the
tissues above the WolfBan bodies, When the kidneys
are formed it receives blood from them, and thencefor-
ward enlarging rapidly eventually becomes the channel
by which the greater part of the blood from the hind limbs
and the hinder part of the body finds its way to the heart.
In proportion as this vena cava inferior increases in size,
and the Wolffian bodies give place to the permanent
kidneys, the posterior cardinal veins diminish. The
blood originally coming to the posterior cardinals from
the posterior part of the spinal cord and trunk is trans-
ported into two posterior vertebral veins; which are
placed dorsal to the heads of the ribs and join the
anterior vertebral veins. With the appearance of these
reins the anterior part of the posterior cardinals dis-
appears.

At its first appearance the vena cava inferior may
be considered as a branch of the trunk wliich we ha^e
called the sinus venosus, but as development proceeds,
and the vena cava becomes larger and larger, the sinus
venosus assumes more and more the appearance of beii^
merely the cardiac termination of the vena cava, and

S86 THE SIXTH DAY. fCHAP.

the ductus veuosua from the liver may now be said to join
the vena cava instead of being prolonged Into the sinus.
While this growth of the vena cava, is going on, the
points at which the ductus Guvieri enter into the sinus
venosus are drawn in towards the heart itaeltj and finally
these trunks fall directly and separately into the auricular
cavities, and axe henceforward knowu as the right and
left vena cava superior (Fig. 90, V.S.R, V.8.L.). There

Diagram of the Vesoub CmotrLATiON DniuNo thb Later
Days of Incdbation.

n. heart V.S,& right vena cava auperior. V.S.L. left vena Ci
Baperior. Tbe two venas cavm superiores ore the original
' ductus Cuviori,' they atill opeu into the ainua venoaua and
not independently into tho heart. J. jugular vein. SU.T.
superior vertebral vein. In. V. inferior vertebral vein,
vein for the wing. V.C.I, vena oava inferior, which reosives
moat of the blood from tho inferior eitremitiea, eto. J3.V,
ductus vecosua. P. V. portal vein. M. a. vein bringing
blooil from the intestines iuto the portal vein. Of. vitelline
vein. ('. allantoic vein. The three last mentioned vein
unite together to form the portal vein. I. liver.
Thu remnantflof the inferior" cardinal veins are not shewn.

THE VEKOUS SYSTEM.

ire therefoTG, when these changes have been effected,
three separate channels, with their respective orifices,
_ which the blood of the body is brought hack to the
Jttrt, viz. the right and left superior and the inferior

Vliile the auricular septum is as yet unformed, the
blood from these veins falls into both auricles, perhaps
into the left than int<j the right. A^ the septum
»we?er grows up, the three vessels become connected
*itii the right auricle only while the left receives the
*W0 pulmonary veins coming from the lungs. (Compare
CIttp. vn. p. 228).

On the third day the course of the vijssela from the
folk-sac is very simple. The two vitelline veins, of
™ch the right is already the smaller, form the meatus
*Wiogus from which, as it passes through the hver on its
•*? to the heart, are given off the two seta of vense
M^ehentes and vonae revehentes.

With the appearance of the allantois on the fourth
'J. a new feature ia introduced. From the meatus
•eoosub, a short distance behind the liver, there is given
* a vein which quickly divides into two branches,
twee, running along the ventral side of the body from
* walla of which they receive some amount of blood,
B to the allantois. They are the allantoic or um-
K*l veins. The single vein which they unite to form
WtueB, by reason of the rapid growth of the allantois,
J long; and hence it ia perhaps better to speak of it
the allantoic vein (Fig. 90, U). The right branch
1 diminishes in size and Bnally disappears. Mean-
iile the left on reaching the allantois bifurcates ; and,
two branches becoming large and conspicuous, there

888 THE SIXTH DAY. [CHAP.

still appear to be two main allantoic veins uniting at a
abort distaace from tbe allantoia to form the single long
allantoic vein. At its first appearance the allantoic
vein seems to be but a small branch of the vitelUae,
but as the allantois grows rapidly, and the yolk-sac
dwindles, this state of things is reversed, and the less
conspicuous vitelline appears as a branch of the larger
allantoic.

On the third day the blood returning from the walls
of tbe intestine is insignificant in amount. As however
tbe intestine becomes more and more developed, it
acquires a distinct venous system, and the blood sent to
it by branches of the aorta is returned by veins which
form a trunk, the mesenteric vein (Fig. 90, M), falling
into the vitelline vein at its junction with the allantoic
vein.

These three great veins in fact, viz. the vitelline,
the allantoic, and tbe mesenteric, form a large common
trunk which enters at once into tbe liver, and which we
may now call the portal vein (Fig. 90, P. V.). This, at
its entrance into the hver, partly breaks up into the
venffl advehentes, and partly continues as the ductus
venosua straight through the liver, emerging from which
it joins the vena cava inferior. Before the establish-
ment of the vena cava iuierior, tbe vense revehentes,
carrying back the blood which circulates through the
hepatic capillaries, joined the ductus venosus close to
its exit from the liver (Fig. 89). By the time however
that tbe vena cava has become a large and important
vessel it is found that the veu£e revehentes or as we
may now call them the hepatic iiein* have shifted their
embuucbment and now fall directly into that vein, the

4nctti8 veDosu3 making a separate junction rather higher
up [Fig, 90).

This state of things continues with but slight changes
till near the end of incubation, when the chick begins
fa) breathe the air in the air-chamber -of the shell, and
iration is no longer carried on by the aUantois.
Bood then ceases to flow along the allantoic vessels ;
ne; become obliterated. The vitelline vein, which as
jolk becomes gradually absorbed proportionately
nminishes in size and importance, comes to appear as
Unwe branch of the portal vein. The ductus venosus
es closed, remaining often as a mere ligament;
tod hence the wbole of the blood coming through the
{portal vein flows into the substance of the liver, and
1 by the two hepatic veins into the vena cava (Fig.
' BP).
Previous to these changes one of the veins passing
n the rectum into the vena cava has given off a
neb which effects a junction with one of the mesen-
c veins. This now forms a somewhat conspicuous
NOMcdng branch between the systems of the vena
in and the portal vein (Fig. 91, Cy. M.).

All three vense cavie now fall exclusively into the
E^t auricle, and by the closure of the foramen ovale
* blood flowing through them is entirely shut off from
" left auricle, into which passes the blood from the
fo pnhnonary veins (Fig. 91, L. V.).

Such is the history of the veins in the chick. As
•ill he seen in the second part of this work, the course
^ents in the mammal, though in the main similar,
ferain some unimportant respects.
It remains for us to speak of the changes which
P, A B. \Sl

THE SLSTH DAY.

DuGRAU or THE V&NOUS CiRCDi^TioN OF TBK Chick abtbb

THE COUMGHOEUENT OF BkbFIBATION BT UEANa OF TBI

W. wiog vein, J, jugular vein. S«. I', superior vertobml vein.
/n, r. inferior vertebral vein. These unite together on each
side to form the oorreaponding auparior vena cava, L.V.
pulmouary voina. V.C.I. vena cava inferior. H.P. bepatio
veina, P. T. portal vein. M. mesenteric veins. Cy.M. oon-
Recting vessel between the branches of the portal vein and
the ajHtem of the vena cava inferior. It is called the cooey-
geo-tneaenterio vein, and unites the cross branch connecting
the two hypogastrics with the mesenteric vein. The ductus
renosuB has become obliterated. The three veme cavie foil
independently into the right auricle and the pulmonary
veins into the loft auricle, Cr. crural velo. k. kidney.
L lirer, pp. hypogastric veiiis. C. V. caudal veju.

have in the meantinie been taking place in the arterial
system. The condition of things which exists on the
Bfth 01 sixth day is shewn in the diagram (Fig, 92).

Fifth or Sixth

'.J, eitemal carotid. I.V.A. iuternal curotiJ. D.A. dorsal
aorta. Of.A. vitelline artery. U.A. allautoic arterj.

We have already seen (Chap, vii. p. 225) that of
™e three aortic arches which make their appearance on
»e third day, the first two disappear : the first on the
■iwth, the second on the fifth day ; but that their dis-
S'pewance ia accompanied by the formation behind
"len of two new aortic arches, the fourth and the fifth.
■""US there are generally three, never more than three,
P^ra of aortic arches present and functional at one time.

Ttijj statcmeot Deeds some limitation ; fur accordiug to Von
*w thara are four arohes present both on the fourth and
' '™' lays. In the case of the fourth day a alight remnant of the
_BBrt paif o[ arches still persiKts when the fourth jiair is already
"I'tted; and on the fifth day the second pair has not entirely
'*Ppaiircd whei) the fifth piiir is formed. In both of these
**• however the firet pair of arches of the four ia only [iroaent
** % Very short time, aiid then is so diminished in size as to be
"'m importance.

S92 THE SIXTH DAY. [CHAP.

The first pair of arches, before it entirely disappeais,
sends off on each side two branches towards the head.
Of these, one forms the direct continuation of the bulbus
arteriosus in a straight line from the point where the
first aortic arch leaves it ; primarily distributed to the
tongue and inferior maxillary region, it becomes the
external carotid (Fig. 92, E.C.A.). The other, starting
from the point where the aortic arch of each side joins
its fellow, dorsal to the alimentary canal, to form the
dorsal aorta, is primarily distributed to the brain, and
becomes the internal carotid (Fig. 92, 1.C.A.).

When the first arch disappears, the external carotid
arteries still remain as the anterior continuations of the
bulbus arteriosus. And since the dorsal trunks uniting
the distal ends of the first and second arches do not
become obliterated at the time when the first pair of
arches disappears, the internal carotids remain aa
branches springing from the distal ends of the second
pair of arches ; they are supplied with blood from that
pair, the stream in which flows chiefly towards the head
instead of backwards towards the dorsal aorta, as is the
case with the succeeding arches. When the second
pair of arches is obliterated, the connecting branch witi
the next arch is again left, and thus the interna! carotids
appear as branches from the distal ends of the third
pair of arches.

On the third day the dorsal aorta does not for any
distance remain single in its backward course along the
body, but soon divides into two trunks which run one
on either side of the middle line of the body. These
two trunks, as development proceeds, gradually unite
along their whole length, and there is thus formed a

CL] THE PULMONARY ARTERIES. 293

angle mediaa aorta terminating behind in the caudal
arteij (figs. 92, 94), The arteries to the kidneys,
liinJ liuibs, etc are developed as brancbea of this aorta.

As the ailantois grows rapidly and becomes an im-
portant respiratory organ, the allantoic or umbilical
Tleries increase in size. As a general, though ap-
Jorently not invariable rule, the right allantoic artery
geU gisdiially smaller and soon disappears.

The vitelline artery {Of. A) now leaves the aorta
*8 a tingU but quickly bifurcating trunk, which at the
Bud of the fifth day is still very large,

By the fifth day the ventricular portion of the heart
(oompare Chap. vw. p. 257) is completely divided into
t*o chambers. The bulbus arteriosus is also divided
hy It septum into two channels, one of which com-
niwiicates with the right ventricle of the heart and the
fther with the lefL

One result of this arrangement is that all the
Mood which passes to the anterior eitremity of the
Wy comes from the left ventricle of the heart

At about the seventh day an entire separation
■^giiis to take place between the arterial roots which
*o>e respectively from the right and left chambers of
"^ Wrt. The root from the right chamber (Fig. 93)
"■JiMns connected with the fifth pair of arches. The
■*>! from the left ventricle is connected with the third
•od fourth pairs of arches.

The lower part of the body still receives blood from
*^ the right and left ventricles, since the blood which
^teia the <ifth arch still fiowa into the common dorsal
***a. As the lungs however increase in size, a com-
ia set up between tfiem and the fifth put of

THE SIXTH DAY.

Fib. 03.
E.C

DiAGBAu OP THE Conhitios of the Abches of the AoB*
TOWARDS THE Close of Incubation.

I, 2, 3, 4, 5, the several aortio atchee. E.C. A. extenuil cBw-
l.C.A. intemftl carotid. CCA. cummon carotid. FaW"
tebral artery. R-tv. right subclftviaii. i.*;. left BubctiTU"'
R.P., L.P. right and left pulmonary arteries. R.P.A. rlgl*
artfirial root or diviaiou of the bulbufl arteriosus, or p'*'
monary artery ; the left root or division, constituting B*
aorta, is seen by its sidu. The system of the fifth wcb i*
in lighter shading. The dotted lines shew the portioM <^
the arches which have been obliterated,

arches in the shape of two vesseb which, springing onO
Irorn the arch of each side, grow dovrawards towards tb'
lungs. At. first small and narrow, these pulmonBiy
arteries, for such they are. grow rapidly lai^er an"-
larger, so that more and more of the blood from Ui*
right ventricle is carried to the limgs.

At the same time the connection between the thi***
imd fourth pairs of arches on each side grows weak*

: CAROTID ARTERIES. 295

iJui less and less of the blood which flows along the
'hird pair of arches is able to pass backwards to the
fiind end of the body.

The fourth arch of the right side now becomes the
meet iiuportaut of all the arches ; and nearly the whole
*f the blood supplying the hinder parts of the body
pftssea through it It is this arch which remains as
the penuanent aortic arch of the adult ; and it is im-
poftaat to notice that the arch which forms the great
wrstti aorta in birds ia the fourth on the right side, and
""^ as in maminais the fourth on the left side. The
''"nh arch of the left aide in birds, after giving off the
Wbclavian, ia continued as an exciiedingly small and
™iinportant vessel to join the fourth right arch. It is
»oa obliterated.

Ia consequence of these changes the condition of
™^ urlic arches during the latter days of incubation,
''™we respiration by the lungs has commenced, is as
^yre (Fig. 93).

The first and second arches are completely ob-
""•sled. The third arch on each side b continued at
«• dor&al end as the internal carotid. I.C.A. the con-
^'^a between it and the fourth arch having become
Oitirely obliterated. From its ventral end as the direct
watinuation of the trunV which originally supplied the
^ Ud second arches the external carotid, E.C'.A., is
flvon off. Each pair of carotids arises therefore from a
""inion trunk — the common carotid {C.C.A.). Each
w these trunks gives off near ita proximal end a branch,
^vertebral artery {V.a.).

The common carotid on the right side comes off
n the fourth arch of the right side (the arch of the

dorsal aorta), and is not as yet uouuected witli ttte lighl
subclavian, R.sc. Tho cummon carotid of the left ak
comes oflf from the fourth arch of the left aide ; but anoe
this arch becomes the left subclavian, L.sc. (the oonnw-
tion between the fourth and fifth left arches bang
obliterated), the portion of the trunk bL'tween the fourtli
arch and the bulbus arteriosus (or as it must nowlw
called the common aortic root) is called the left
innominate artery.

The fourth arch of the right aide forms the com-
mencement of the great dorsal aorta, and gives off the
right subclavian (R.sc.) just before it is joined by the
fifth arch.

The fifth arch of each side gives off branches {RS-,
L.P.) to the lungs ; their distal continuations, by whid*
these arches are connected with the systemic circuUtioCt
though much reduced, are not obhterated.

The final changes undergone by the arterial syiitan
after the commencement of the pulmonary respiiatjoo
consist chiefly in the complete separation of the pul'
monary aud systemic circulations. As the branches V>
the lungs become stronger and strongtr, less and les»
blood from the right ventricle enters into the doiw*
aorta; and the connecting vessels become smaller ai"
smaller.

Each of these fifth arches from the right ventricl*
may therefore be considered at about the sixteentb or
eighteenth day as divided into two parts, an inner part
which connects the heart with the lung, and an ouW
part which still connects the arch with the mcun doiw
aorta. As these outer parts become smaller they re-
ceive the name of the 'ductus or canales BotalU' "^

U-] SHMMAHY. 297

■ductus arteriosi.' The one on the right side is short ;
wat on the left side w much longer and narrower.

When respiration commences the blood ceases to
pMs through these canals, which either remain as mere
"goJaents or else become absorbed altogether. By this
"'^sos, the foramen ovale becoming at the same timu-
t^sed, a complete double circulation is established. All
"■e blood from the right ventricle passes into the lungs,
Wii all that from the left ventricle into the body at

Two other changes take place about the same time
ttie aortic branches. That portion of the right fourth
*aortic arch which lies between the origin of the right
'"''clavian and the common carotid becomes shortened,
7"* is finally swallowed up in such a fashion that the
"S'^t subclavian (Fig. 94, R. sc.) comes off from the
'pit common carotid, a very short trunk being formed
the union of the two to serve as the right innomi-
B artery.

-^t the same time, corresponding to the increase in
^ length of the neck, the common carotids are very
S'^^tly lengthened. They lie close together in the
'^^t, and in many birds actually unite to form a com-
"'** trunk.

Jt will of course be understood that with the dis-
ice of the allantois and the absorption of the
:, the allantoic and vitelline arteries also disappear.
It may perhaps be of advantage to the reader if we
briefly summarize the condition of the circulation
its four most imptortant epochs ; ^
ly, on the fifth duy, during the later days (
before lespiiation by the lungs has c

he cu^^ulation
on the thi^^^H
days of i^^^^^^l

THE SIXTH DAT.

DlASRAM OF T

cau-

B Artbulaj. Ststem of t

I Adolt Fo"-

P.A. root of pulmonftry artery. L.in. left imiominatf »!*!■
i).J. dorsal aorta. Or. cieliac arteries. 7na». mtwt'l''
artery, ar.r. renal arteries, fem. femoral artehec ^
isoLiatic artoriea. h^p. hypogaBtric arteries, con. W*^
artery. The other letters as in Fig. 93.

and after the chick haa begun to breathe by t*
lungs.

On the third day the circulation is of an ej
iqgly Biinpie character.

n.] suiQfAKT. 299

The heart k to all intents and purposes a simple
twisted tube marked off by constrictions into a series of
three ccmsecutive chambers. The blood coming finom
the venous radicles passes through the heart and then
through the three pairs of arterial arches.

From these it is collected into the great dorsal
aorta. Upon this dividing into two branches, the stream
of blood passes down on each side of the notochord
along the body, and thence out by the vitelline arteries,
which distribute it to the yolk-sac

In the yolk-sac it partly passes into the sinus termi-
nalis and so into the fore and aft trunks, partly directly
into the lateral trunks, of the vitelline veins. In both
cases it is brought back to the two venous radicles and
80 to the heart

On this day the blood is aerated in the capillaries of
the yolk-sac.

On the fifth or sixth day the two auricles are
present though having a common cavity. The septum
of the ventricles is nearly complete, so that the blood
on entering the ventricles from the auricles is divided
into two streams. These two streams pass respectively
from the right and left chambers of the heart into the
two divisions of the bidbus arteriosus. The blood from
the right ventricle passes into the fifth pair of arches
and that from the left ventricle into the third and
fourth pairs of arches.

From the anterior parts the blood is brought back
by the anterior cardinal or jugular veins; from the
hinder parts of the body, chiefly by the cardinal veins,
but also in part by the now commencing vena cava
inferior.

800 THE SIXTH DAY. [cHAP.

The blood from the yolk-sac and allantois, leather
with a smalt quantity from the intestine, is collected
into the portal vein, and by that vessel carried to tha
liver. Here it becomes divided into two streams, part
flowing directly by the ductus venosus into the sinus
venosus, and the remainder passing through the capil-
laries of the liver, being brought back to the ductus
venoaus by the hepatic veins.

During this period the blood is aerated both by the
allantois and yolk-sac, but aa yet chiefly by the latter.

At a somewhat late period of incabation the
blood from the ventricles passes into two entirely dis-
tinct roots. The one of these, that from the right
chamber, sends the blood to the fifth pair of arches;
passing through which the greater part of the blood
flows into the dorsal aorta, a small portion only finding
its way into the lungs through the as yet unimportant
pulmonary arteries.

Through the other aortic root, viz. that from the
left ventricle, the blood flows into the third and fourth
pairs of arches. That part of the blood which flows
into the third pair, passes almost entirely to the head
and upper extremities by the external and internal
carotids ; that which flows inU.i the right arch of
the fourth pair is chiefly brought to the dorsal aorta, ■
but some of it passes to the right wing ; that, on the
contrary, which goes into the left fourth arch is for the
most part sent to the left wing, a small part only reach-
ing the dorsal aorta. There is still a mixture of the
blood from the two chambers of the heart, so that the
blood in the dorsal aorta is composed partly of blood
Ajm the left, and partly from the right chambers.

Uj SUMMARY. 301

I Itlood of the upper (anterior) end of the body

mmtirelj from the left ventricle.

Ute bLxxi of the ilorsal aorta passes to the jolk-
•c and aJlantoia, and to all the hinder parts of the
kodj. It is brought back from the yolk-sac, from the
ilUntois, and to a certain extent from the intestines, by
the porta! vein, part of the blood from which passes to
the inferior vena cava by the direct course (ductus
'enogos), and part indirectly by the more circuitous
•"use of the capillaries of the liver and hepatic veins.

The blood from the generative and urinary organs,
•M from the hinder extremities, is brought back to the
li^rt by the vena cava inferior ; that from the upper
tttremities and head by the jugular, vertebral and
*iDg veins into the two vense cavse of the right and

Bide, and so to the heart. Of these three vense

I, the right superior and the inferior join the
*>Tie!e by a common entrance, but the left superior
liu an entrance of its own. All of these open into
•lie cavity of the right auricle, but the opening of
"» inferior vena cava is so directed {vide Chap. vm.
P 263) that tlie blood carried by this vessel flows
•liiefiy through the foramen ovale into the left auricle,
"e blood from the two superior venas cavas enters the
"jglit auricle only. Now the blood of the inferior
'Ku cava has been partly aSratud by the allantois;
""i. since it is this blood which passing through the
"ft auricle and ventricle is distributed to the third
"ttic arch, unmixed by any blood from the right ven-
''icle {the mixture with the blood from the fifth arch
**ching only as far as the fourth arch), it happens

the blood which flows to the anterior extiemltJes

302 THB SIXTH DAV. [CHU.

and head is more aerated than that in aoj otbei {art
of the body.

From tho anterior extremities the blood is lo •
great extent returned by the left superior cava, iU"
goes into the right auricle, whence, by the right ven-
tricle, it is distributed tlirough the fifth pair of wchei
over the body, after joining the more aeivted blod
passing through the fourth pair of arches.

Tlie blood from the lungs is brought back bj W
small veins into the left auricle.

The characteristics of the circulation at this tJnwaB
that the blood is aeratad by the allantois, and that lhe»
is a partial double circulation. (Vide Chap. TiiLp,263.1

As soon as respiration commences the cioi^
leading to the dorsal aorta from the fifth pair of arcbt*
which communicate only with the right ventricle, 1*"
come closed. The blood passing along the fifth tt"
now flows only into the lungs, through the pulmoiBiJ
arteries. The blood from the left ventricle owing "
the cessation of the circulation of tlie yolk-sacandoftW
allantois is distributed exclusively to the body of lb*
chick, from whence it is all brought back into the iig»
auricle by the three now independently opening
cavEB.

The portal veins henceforward receive blood
the intestines only, and the ductus veuosus is o*"
literated, so that all the blood of the portal vein ;
through the capillaries of the liver.

The partition between the auricles is rendered o*'
plete by the cIosutg of the foramen ovale ; into t*
right auricle the veins of the body enter, and inW '■
left the pulmonary veins.

bl] hatching. 803

There is thuB a completely doable dnnilAlioii fbmied,
in which all the blood of the left Tentxide is arterial,
and all the blood of the ri^t ventricle renoua, and
there is at no part of the drculation a mixture of venous
and arterial blood.

As early as the sixth day movements, as we have
said, may be seen in the limbs of the foubryo upon
opening the egg. We may conclude that after this
epoch spontaneous movements occur from time to time
in the unopened egg. They cannot however be of any
great extent until the fourteenth day, for up to this
time the embryo retains the position in which it was
first formed, viz. with its body at right angles to the
long axis of the egg.

On the fourteenth day a definite change of position
takes place ; the chick moves so as to lie lengthwajrs in
the egg, with its beak touching the chorion and shell
membrane where they form the inner wall of the
rapidly increasing air-chamber at the broad end (Chap.
L p. 3).

On the twentieth day or thereabouts the beak is
thrust through these membranes, and the bird begins
to breathe the air contained in the chamber. There-
upon the pulmonary circulation becomes functionally
active, and at the same time blood ceases to flow
through the umbilical arteries. The allantois shrivels
up, the umbilicus becomes completely closed, and the
chick piercing the shell at the broad end of the egg
with repeated blows of its beak, casts oflF the dried re-
mains of allantois, amnion and chorion, and steps out
into the world.

INTRODUCTION.

The most important difference between the de-
velopment of Mammalia and Aves depends upon the
amount and distribution of the food-yolk in the ovum.
In birds, as we have Been (Ch. I.), the ovum is large and
the greater part of it so heavily charged with food-yolk
that it is unable to segment. The segmentation is con-
fined to one small portion, the germinal disc, the pro-
toplasm of which is less burdened with food-yolk than
that of the remainder of the ovum. Such partial seg-
mentation is known as Tneroblastic.

In Mammals, on the other hand, the ovum is small*,
and contains but a slight amount of food-yolk ; the little
there is being distributed uniformly throughout. In con-
sequence of this the whole ovum is able to segment ; the
segmentation therefore belongs to the holoblasHc type.
This fundamental difference in the constitution of the
ovum of Birds and Mammals is accompanied not only by
differences in the segmentation but also by impoitant
differences, as we shall see, in the stages of development
which immediately follow segmentation. Finally, in

I The human c

^iiof"

INTRODUCTIOK.

birdB, as we have seen, the nutrition of the developing
8 entirely effected at the expense of the food-
yolk and albumen with which the ovum was charged
in the ovary and oviduct respectively, and the eggs
leave the parent very soon after the close of segmenta-
tion. In the Mammalia the absence of sufficient food-
yolk necessitates the existence of some other source ot
nutriment for the embryo, and that source is mainly the
maternal blood.

The development of Mammalia may be divided into
two periods : 1. the development within the uterus ; 2.
the development after birth.

In all the higher Mammalia the second period is very
unimportant, as compared with the first ; for the young
are bom in a condition closely resembling that of the
adult of the species to which they belong. The de-
velopment during the first period takes place in the
■ uterus of the mother, and nutriment passes from the
maternal blood to that of the embryo by means of a
structure, to be described in detail hereafter, known as
the placenta. This difference between the development
of Birds and Mammals may be brieiiy expressed by saying
that the fonner are oviparous, while the latter are vivi-
parous.

The source of nutriment during the second period
is the Mammary glands. In certain of the lower Mam-
malia (Marsupials) the young are_ bom in a very im-
mature condition, and become attached by their moutlis
to the nipples of these glands. They are carried
about, usually in a special pouch (marsupium) by the
mother, and undergo in tliis position the greater part
the remainder of their development.

CHAPTER X.
6EKEBAL DEVELOPMENT OF THE EMBRYO.

There is a close agnaement in the history of the
development of the embryo of the various kinds of
Mammals. We may therefore take one, the Rabbit, as
a type. There are without doubt considerable varia-
tions to be met with in the early development even of
species nearly allied to the Rabbit, but at present the
true value of these variations is not understood, and
they need not concern us here.

The ovarian orum. Mammals possess two ovaries
situated in the body cavity, one on either side of the
vertebral column immediately posterior to the kidneys.
They are somewhat flattened irregularly oval bodies, a
portion of the surface being generally raised into pro-
tuberances due to projecting follicles.

In an early stage of development the follicle in the
mammalian ovary is similar to that of the fowl, and is
formed of flat cells derived from the germinal cells ad-
joining the ovum. As development proceeds however
it becomes remarkably modified. These dat cells sur-
rounding the ovum become columnar and then one or
two layers deep. Later they become thicker on one
side of the ovum than on the other, and there appears

310 THE MAMMALIAN EMBRYO. [CHAP.

iu the thickened mass a cavity which gradually becomes
more aod more distended and filled with an albuminous
fiuid.

As the cavity enlarges, the ovum, around which are
several layers of cells, forms a prominence projecting
into it. The follicle cells are known as the membrana
granulosa, ajid the projection in which the ovum Hes as
the discus or cumulus proligerus. The whole structure
with its tunic is known as the Graafian follicle.

If the ovary of a mature female during the breeding
season be examined, certain of the protuberances on its
surface may he seen to be considerably larger than others ;
they are more transparent than their fellows and their
outer covering appears more tense ; these are Graafian
follicles containing nearly or quite ripe ova. Upon pierc-
ing one of these follicles with a needle-point the ovum
contained therein spirts forth together with a not incon-
siderable amount of clear fluid.

Egg Hembranes. The ovum is surrounded by a
radiatc'ly striated membrane, the zona radiata, internal
to which in the nearly ripe egg a delicate membrane
has been shown, by Ed. v. Beneden, to esist. The cells
of the discus are supported upon an irregular granuJar
membrane external to the zona radiata. This mem-
brane is more or less distinctly separated from the zona,
and the mode of its development renders it probable
that it is the remnant of the first formed membrane
in the young ovum and is therefore the vitelline mem-
brane.

Maturation and impregnation of the ovum. As

the ovum placed in the Graafian folhclo approaches
maturity the germinal vesicle assumes an excentiic

IMPREGNATION.

311

»^i

a series of changes which have
out, but which prohably arc of
lae which have been observoi.! in
The result of the changee is the
more polar bodies, and the nucleus
Dale pronucleus).
le or more follicles containing ;i
their contents are received by
unity of the Fallopiao tube which
HenseQ to clasp the ovary at the
:r the exit of the ovum becomes
remains as a conspicuous object on
ovary for some days. It becomefi
luteum. The ovum travels slowly
tube. It is still invested by the
the rabbit an albuminous envelope
■ in its passage downwards. Im-
in the upper part of the Fallo-
Jy followed by the segmentation.
amongst the Amniota for being

the spermatozoon into the ovum

i b,te have not been observed. Van

I the rabbit the formation of the

i nucleus (Le. the nucleus of the ovum

I from two nuclei, one penpht-ral and

\ and deduces from his observations

e u Qo relatioD between the bnntiuK of

,t iliurlly »llei iajpregnatiu», viA
egmeDtation, I))B ova nt iliv m>iMl
Us Mid exhibit Out fhenoaif
•d trj other Dbaorvsn.

81S

THE HUIMALIAN EMBRYO,

[chap

that the peripheral nucleus was derived from the aper-
matic clement.

Segmeotation. The process of segmentation oc-
cupies in the rabbit about 72 hours; but the time of
this and all other stages of development varies con-
siderably in different animals.

The details of segmentation in the rabbit are differ-
ently described by various observers ; but at the close of
segmentation the ovum appears undoubtedly to be
composed of an outer layer of cubical hyaline cells,
almost entirely surrounding an inner mass of highly
granular rounded or polygonal cells

OmoAi,

A RABBIT'a OVOM At TWO SUOH
OLOSBLT FOLLOWING UPON TEB SSOIIEHTATIOR.

(After E. van Beueden.)

ep. outer lajei' ; Ay, inner mass ; bj/. Van Benuden's blastopore.

The shading of t!ia outer aiid inner layeiB is diagrammatio.

In a small circular area however the inner
ceJIs remains exposed at the suriace (Fig. 95,

er masB of ^|

SEGMENTATION.

exposed spot may for convenience be called with v. Bene-
den the blastopore, though, as will be seen by the ac-
count given of the subsequent development, it in no
way correspond'i with the blaatopore of other vertebrate

111 the following account of the segmentatioa of the rabbit's
ovum, V. Benedea'a description is followed lu far as the details
are concerned, hia Domenclature is however not adhered to'.

According to v. Beneden the ovum fint divides into two
nearly equnl Bpheree, of which one is slightly larger and more
transparent than the other. The larger sphere and its produota
will he spoken of as the outer spheres, and the amaller one
and ita products as the inner apberea, in accordance with their
diSerent destinations.

Both the spheres are soon divided into two, and each of the
four so formed into two again ; and thus a stage with eight
spheres ensues. At the moment of their first separation these
spheres are spherical, and arranged in two layers, one of them
formed of the four outer, and the other of the four inner spheres.
This position is not long retained, for one of the inner sphsTee
paaseii to the centre ; and the whole ovum again takes a spherical

In the next phase of segmentation each of the four outer
spheres divides into two, and the ovum thus becomes constituted
of twelve spheres, eight outer and four inner. The outer spheros
have now become markedly smaller than the inner.

The four inner spheres next divide giving rise, together with
the eight outer spheres, to sixteen spheres in all ; which are
nearly uniform in siae. Of the eight inner spheres four soon
pass to the centre, while the eight now superficial outer spheres
form a kind of cup partially enclosing tho inner spheres. The
outer spheres now divide in their turn, giving riss to sixteen

^H ^ Theci

^^^ epiblsBt, wl

■I

The cuUb Bpoki
epiblsBt, whilst those

□faathem

Bspond to Van Beneden's
innor correspand to his

314 THE MAMMALIAN EMBRYO. [iHAP.

spheres which largely eocloae the iiuer dpherea. The wgmmto-
tioa of both outer and iiiner spheres contimiee, and in the oouh
uf it the outer epheres spread further and further over th« una.
HO that at the close of segmeutatiuu the inner spheres constitDtei
central solid maaa almost entirelj surrounded by fha outei
spliercB. In a small circular area however the inner dub rl
spheres remain for some time esposed ot the surfece (Fig. S5 A),

The blastodermic vesicle. After its aegmenMioii
the ovum passes into the uterus. Tbe outer ceUa soon
grow over the blastopore and thus form a complete
superficial layer. A series of changes next take plwf
which result in the formation of what has been called
the blastodermic vesicle.

These changes commence with the appearance of »
narrow cavity between the outer and inner layers, which
extends so as completely to separate them except in th*
region adjoining the original site of the blastopore (Fig.
95 B)'. Tbe cavity so formed rapidly enlarges, ad
with it the ovum also ; so that this soon takes the foro
of a thin walled vesicle with a large centnd cavitf
This vesicle is the blastodermic vesicle. The gre«W
part of its walls are formed of a single row of flattenfid
outer layer cells; while the inner mass of ceils fonw
a smalt lens-shaped mass attached to the inner side <^
the outer layer (Fig. 96).

Although by this stage, which occurs in the ratt*'
between seventy and ninety hours after impregDatioB,
the blastodennic vesicle has by no means attained if
greatest dimensions, it has nevertheless grown frfflS

Vuu Benedcn regards it as probftbla that the bUstopolt *
situated somuwbBt eieeatricollj in relation to the area of
of the inner mass to the outer la^ei.

BLASTODERMIC VESICLE.

boat O09 mm. — the saze of the ovum at the close i
^mentaLion — to about 0'28 in dimneter. It is en-
baed by the zona radiata and thi? albui

'0—90 HOCBS AFTER iMl'KEONATIO.S.

(After E. ran Beueden.)

r of blastndenuio veaiate (jolk-sac) ; ep. ciuUtr layer ;
ky. inner mass ; 2^. albuminous envelope.

it. The blastodermic vesicle continues to
rapidly, and during the process the inner nutsB
i^^oee important changes, It spreads out <:
r ode of the outer layer and at the same time \i
]«ne-Uke form and becomes flattened. The c

814 THE MAMMALIAN EUBRTO. [CHAP.

part of it remains however thicker, and is constituted
of two rows of cellfl, while the peripheral part, the outer
boundary of which is irregular, ia formed of an imperfect
layer of amceboid cells which continually spread further
and further beneath the outer layer. The central thick-
ening of the inner layer forms an opaque circular spot
on the blastoderm, which constitutes the commencement
of the embryonic area.

The formation of the layers. The history of the
stages immediattily following, from about the com-
mencement of the fifth day to the seventh day, when a
primitive streak makes its appearance, is not perfectly
understood, and has been interpreted very differently by
various observers. The following account must there-
fore be considered as a tentative one.

About five days after impregnation the cells of the
inner mass in the embryonic area became divided into
two distinct strata, an upper stratum of rounded cells
adjoining the flattened outer layer and a lower stratum
of flattened cells. This lower stratum is the true hypo-
blast (Fig. 97). At the edge of the embryonic area the
hypoblast is continuous with a peripheral ring of the
amcnboid cells of the earlier stage, which now form,
except at the edge of the ring, a continuous layer of
flattened cells in contact with the outer layer. During
the sixth day the middle layer becomes fused with the
outer layer, and gives rise to a layer of cells which are
columnar and are arranged in the rabbit in a single
row (Fig. 98), They form together the true epiblaat of
the embryonic area.

At this stage therefore the embryonic area, which is
circular, is formed throughout of two single layers of

X.]

FORMATION OF THE LAYERS.

317

cells, a columnar epiblast and a layer of flattened hypo-
blast.

Pio. 97.

SsOnON THROUGH THE NEARLY CiRCnLAR EmBRTONIO ArBA OF

A RABsrr Ovum of Six Datb.

(From Allen Thomaon, after E. van Beneden.)

0cf . upper layer ; me^, middle layer ; ent, true hypoblaiit.

Fio. 98.

Skotion through the Blastoderm of a Rabbit on tbs
SsTENTH Day : taken in front of the Primitive
Streak.

Half of the area in repreflented.

Towards the end of the sixth day the eiiibryonio
area of the rabbit, which has hithert^i lx;en round, bn-
comes oval.

A diagrammatic view of the whole blontodorinia
vesicle at about the beginning of the m.'Vijnth day ii
given in Fig. 99. The embryonic an^ in rfiimmmUnl in
white. The line ge in B shows the f.xU'unutu of tht
hypoblast round the inside of thi; vc^Kick. 'VVv^

Views of the JJlabtodkkuic Vbbiolb of a itiiiiiiT on tbe
Sbvenih Day withodt the Zona. A. from above, B.
from the si.lt. {From Kolliker.)
ag. embryonic urea ; fie. boundiiry of the hypoblAat

X.] PRIMITIVE STREAK 319

todermic vesicle ia therefore formed of three areas,
(1) the embryonic area with two layers, a coluiniiar
epiblast and flat hypoblast ; (2) the region around the
emhryonic aiea where the walla of the vesicle are formed
of flattened epiblast' and of hypoblast ; (3) the area
beyond this again where the vesicle is formed of flat-
tened epiblast' otdy.

The changes which next take place begin with the
formatioii of a primitive streak, homologous with, and in
most respects similar to, the primitive streak in Birds.

Embryonic Area op an Eiobt Dayb' Babbit,
(After Kolliker.)
arg. embryonic area ; pr. primitive streak.
The formation of the streak is preceded by that of a
dark spot near the middle of the blastoderm, forming
the nodal point of Hensen. This spot subsequently
constitutes the front end of the primitive streak.

Early on the seventh day the embryonic area be-
comes pyriform, and at its posterior and narrower end

' The epiblast o( tbe blaBtodennic veBJole beyond the smbiyonic
M [onned of the outer l&jer onlj^.

320 THE MAMMALIAN EMBRYO. [CHW.

the primitive streak makes ite uppearauce ; it is due ui
a, proliferation of rounded cells from the epiblast

Fio. 101,

P' ep

Seciioi' tbhouob an Oval Blastoderm or a Babbs d<
THE Seventh Day, Thk Lenrth ok thb Abu •"•

ABODT 1'2 MM. AND ITS BbBADTH ABOUT "86 MM.

Through the front part of the primitive streak ; ep. ep'bll'ti
m. nmaoblast ; hy, hypoblast ; pr, primitive streak.

These cells give rise to a part of the mesobUsW
layer of the embryo, and may be termed from thar
origin the primitive 8tre!ik mesoblast.

During the seventh day the primitive streak be-
comes a more pronounced structure (Fig. 101), ^
me3Dbla,st in its neighbourhood Increases in quantity,
while ail axial groove (Fig. 100) — the primitive gr(Xi»e
— is formed on its upper surfact.

The formation of the medtJlary groove. In the
part of the embryonic area in front of the priiniti"!
streak there arise during the eiglith day two fol^
bounding a shallow median groove, which meet in ffonV
but diverge behind, and enclose between them tl*
foremost end of the primitive streak (Fig, 103). TbM
folds are the medullary folds and they constitute lb
tirst definite traces of the embryo. The medulliuy pl»l
bounded by them rapidly grows in length, the prinutiTS
streak always remaiuing at its binder end. While tii*

THE MESOBLASI.

Fw. lot.
A.

321

Two Trahstkbse Ssctionb thbough the Embryonic Abba
OF AS Embbto Babbit of Sbyzn Datb.

The embryo has nearlj the appearance lepresoited in fig. lOOl

A. is taken Uuroogh the anterior part of the embrytmic areiL
It represents about half the breadth of the area, and there is no
trace of a medullary groove or of the mesoblast

B. is taken through the posterior part of the primitiTe
streak.

ep. epiblast ; Ay. hypoblast

lateral epiblast is formed of several rows of cells, that of
the medullary plate is at first formed of but a single
row (Fig. 104, mg).

The mesobla^ and notochorcL The mesoblast in
mammalia has, as in the chick, a double origin, and the
details of its development appear to resemble essentially
those in the chick. It arises (1) firom the epiblast of
the primitive streak ; this has been already described ;
(2) from the primitive hypoblast in firont and at the
sides of the primitive streak. The latter is known as
hypoblastic mesoblast, and as in the chick appears to
originate as two lateral plates split oflF from the primi-
tive hypoblast. These two plates are at first continuous
F. & B. ^X

EuBBiosio Area of a Seven Days' EImbbto KabK''

(From Kdlliker,)

0. place of fature area vaaculosa ; rf. nieduUary groove i J»- P"

mitive atreak j ag. embryonic area.

In the region o. a layer of meeoblast has already gMini ; ">*

an.- however as yet no eigas of blood- veesele in iL

Tbia mi:sobIaat is derived &om the mesoblaet of the [Ai^^
streak (Kulliker).

in the axial line with the primitive hypoblast. WW
the medullary groove is formed the lateral hand* "
meBoblast become separate from the axial hypoblast W"
give rise to two independent lateral plates of

THE PRIMITIVE STREAK,

(Fig. 104). The axial band of hypoblast eventoally
gives rise to the notochord.

Trahsvebsx Sectiob

r/,. e[)ibla«t ; Hie. meaoblaat i hy, hypoblast ; in^. mediilUtry

The meaoblastic elements from these two sources,
though at first characterised by the difference in the
appearance of their cells (Fig. 102, B), those of the
primitive streak mcsoblast being more rounded, soon
become blended and indistinguishable from one another;
BO that it is difficult to say to what parts of the fully
formed mesoblast they severally contribute.

In tracing the changes which take place in the rela-
tions of the layers, while passing from the region of the
embryo to that of the primitive streak, it will be con-
venient to follow the account given by Schafer for the
guinea-pig, which on this point ia far fuller and more
satisfactory than that of other observers. In doing bo
Tve shall leave out of consideration the fact that the
layers in the guinea-pig are inverted. Fig. 105 repre-
sents a series of sections through this part in thi- guinea-
pig. The anterior section (D) passes through the medul-
lary groove near its hinder end. TIil' commencement of
the primitive streak is marked by a slight prominence OD
the floor of the medullary groove between the two ^i*!-^-

321 THE UAILMALIAN EMBRYO. [CHAr.

ijig medullary Iblda (Fig. 105 C, as). Where this prami-
nence becomes first apparent the epiblast aod hyjwbUrt

A SEnrES of Traksvbrbe Sections through Tffl! JnrB"''

OF THE pRDOTrVB SlREA-K AND MKD0LLAKT QBiW "

A YoONQ GniNEA-Pxo. (After SohSfer.)
A. ia the poaterior section.

e. epiblast ; m. iiiesoliliwt ; A. lijT>oblaBt ; ac. axial ^P^.
the primitive streak; ah. ftiinl hypoblast attached in I'''^
C. to the epiblast at the rudimentary blaatopwe I "^ "
diiUiirj- groove ; / rudimentary blaatopore.

JL] tee IKfTOCBCOBSL ^SS

are united toge&ec The ineaoUast plaetes «t the t)ii^
adee renudxi in ihe meantixDe quite free. Sl^^vdjr
fiirdifir badL, bat befcre the pnznrtm groove is reftdMid,
the efStAMSt azid li jpoUast ure ooDiieeted toges&er V^ ji
cord of oeOs (Fig. 105 B, jT), idiicii in die section ii@tt
Iblkrviiig becames detaciied from "Ae hjpdblbi^ 4aMl
fomis a solid keel projeetang fircnn tbe epihlbist In lli^
f<dlowiiig fiectkxn t^ hitherto independent SMSobboit
plates beoome united with this keel (F%, 105 A.); aai4
in the post^or sections, throiogh the put of the piUMi*-
tive streak with the pdnutire groove, the epibU^ and
mesoUast continae to be united in the axial line> ))i«t
the hypoblast remains distinct These pecoliar reIali<Mis
may shcnlly be described by saying that in the axiil
line ibe hypoblast becomes unUed wiA Ae qsAUuH «t
the poderior end of the embryo; and that th« celt$
which connect the hypoblast and epblast are posterioify
continuous with the fused epiblast and mesobl^st of
the primitive streak, the hypoblast in the ix^on of tho
primitive streak having become distinct from the otiior
layers.

The notoehord. The thickened axial portion of tho
hypoblast in the region of the embryo becomes scpa«
rated, as we have already pointed out, ftom the latcnid
parts as the notoehord.

Very shortly after the formation of the notoehord,
the hypoblast grows in from the two sides, and becomes
quite continuous across the middle line. The formation
of the notoehord takes place from before backwards;
and at the hinder end of the embryo it is continued
into the mass of cells which forms the axis of the primi*
tive streak, becoming therefore at this point con.titL>x^>^

386 TUE MAMMALIAN EMBRYO. [CBiP.

with the epiblast. The notochord in fact behaves eiactJy
as did the axial hypoblast in the earlier staf^.

The peculiar relatione just mentioned are preciBsl; BimOtfti:
those we have already described in the chick (p. 60). Tbm
receive their explanation by compariBon -with the lower tjp«

The cells which form the junction hetneen the epibUst and
the oiinl hypoblast constitute in the lower tjpea the front mil ot
a passage perforating the blastoderm and leading from the ul-
terior into the alimentary canal. TSiis passage is the rertekite
blastopore.

In the chick we have seeu (p. 12) this pasaoge is present al a
certain stage of devoloproeiit as the neiirenteric canal ; and b I
duck at a still earlier stage. It is also present at an early sU
in the mole.

The presence of this blastopore renders it clear thiit the b1
topore discovered by Ed. van Beoeden cannot have the i
be assigned to it in oomiiariDg it with the blaatopon of U
frog.

To recapitulate. At the stage we have now reacherf
the three layei's are definitely established.

The epiblast is derived partly from the outer lays'
of segmentation spheres and partly from the larger ym-
portion of those segmentation spheres which conatitata
the inner mass. The hypoblast arises from the fe»
remaining cells of the iimer mass ; while the megoblut
ha3 its origin partially from the epiblast of the prijuitiw
streak and partially from the hypoblaet cells anterior to
the primitive streak.

During the period in which these changes have lieen td^
place, the rudimeuia of n vascular area become formed, sad lUSt
aa Keiliker has shewn, tlie mesoblast of thia portion is to son
extent derived from tha mesoblast of the primitive streak, it
poaaible that a portion of it owes its origin to hypoblaatio EW*
blaat

S..'^ THE MEDULLARY PLATE.

327 I

General growth of the embryo. We liave seen

Hiat the blast (xlennic vesicle becomes divided at an

•^arly stage of developmeut into an enibrjonic area, and

' non -embryonic portion. The embryonic area gives

fise to the whole of the bcwly of the embryo, while the

"on-embryonic part forms an appentlage known as the

"•ubiticftl vesicle, which becomt's gradually folded off

from the embrj'o, and has precisely the rehitions of the

if'^lli-Bac of the chick. It is almost certain that the

■™-^jnmaha axe descended from ancestors, the embryos.

*** which bad large yolk-sacs, but that the yolk has

^^'iome reduced in quantity owing to the nutriment

^'^^ived from the wall of the uterus taking the place

" that originally supplied by the yolk. A rudiment of

r*^ yolk-sac bfing thus retained in the umbilical vesi-

'^, this stnictiire may be calKid indifferently umbilical

^^cle or yolk-sac.

The yolk which fills th^ yolk-aac in Birds is re-
t**«ced in Mammals by a eoagulable fluid; while the
fttadnal extension of the hypoblast round the wall of
^Qe blastodermic vesicle, which has already been de-
scribed, is of the same nature as the groivth of the hy-
poblast round the yolk-sai.' in Birds.

The whole embryonic area would seem to be em-
ployed in the formation of the body of the embryo. Its
long axis has no very definite relation to that of the
L blastodermic vesicle. The fijst external trace of the
Bsmbiyo to appear is the medullary plate, bounded by
r the medullary folds, and occupying at first the anterior
half of the embryonic area (Fig. 103). The two me-
dullary folds diverge behind and enclose the front end
^j^ the primitive at^j^^A&^fi embryo elongates the

32S THE MAMMALIAN EMBRYO. [CEU,

medullary folds nearly meet behind and so cut off the

front portion of the primitive Btreak, whicH then ^
pears as a projection in the hind end of the meduUsij
groove, At the hind end of the medullary groova
(mole) a deep pit perforates its floor and enters the
mass of mesohlaat cells lying below. The pit is a rudi-
ment of the blastopore (described on p. 326) which has
been enclosed by the medullary folds.

Henceforward the general course of development is
very similar to that in the chick and so will be only briefly
described. The special features in the development of
particular organs will be described later. In an embiyo
rabbit, eight days after impregnation, the medolUiJ
groove is about 1'80 mm. in length. At this aUge »
division may be clearly seen in the lateral plates of
mesoblaat into a vertebral zone adjoining the emliijo
and a more peripheral lateral zone ; and in the verte-
bral zone indications of two somites, about 0'37 mtt
from the hinder end of the embryo, become apparent
The foremost of these somites marks the junction, w
very nearly so, of the cephalic region and trunk. The
small size of the latter as compared with the former i»
very striking, but is characteristic of Vertebrates gene-
rally, The trunk gradually elongates relatively to
head, by the addition behind of freah somites,
embryo has not yet begun to be folded off from
yolk-sac.

In a slightly older embryo of nine days there s^JpaiCi
(Henaen, Kolliker) round the embryonic area a delicst^
clear ring which is narrower in front than behind (Kj
106 A. ap). This ring is regarded by these authora >
representing the peripheral part of the area pell'icida '

THE CEREBRAL VESICLES.

329

Birds, which does not become converted into the body
of the embrj"0. Outside the area pelhicida, an area
vasculosa has become very well defined. In the em-
bryo itself (Fig. 106 A) the disproportion between head
and tnmk is leas marked than before; the medullary
plate dilates anteriorly to form a spatula-shaped ce-
phalic enlargement; and three or four somites are
established. Tu the lateral parts of the mesoblast of
the head there may he seen on each side a tube-like
structure {hz). Each of these is part of the heart, which
arises as two independent tubes. The remains of the
primitive streak (pr) are still present behind the me-
dullary groove.

In somewhat older embryos (Fig, 106 E) with about
eight somites, in which the trunk considerably exceeds
the head in length, the first distinct traces of the
folding off of the head end of the embryo become ap-
parent, and somewhat later a fold also appears at the
hind end. In the formation of the hind end of the
embryo the primitive streak gives rise to a tail swelling
and to part of the ventral wall of the post-anal gut. In
the region of the head the rudiments of the heart [k)
are far more definite. The meduUaiy groove is still
open for its whole length, but in the head it exhibits a
series of well-marked dilatations. Tlie foremost of
these {vh) is the nidiment of the fore-brain from the
sides of which there project the two optic vesicles (a6) ;
the next is the mid-brain (mh) and the last b the hind-
brain (kh), which is again divided into smaller lobes by
successive constrictions. The meduUarj' groove behind
the region of the somites dilates into an embryonic
smus rhomboidalis like that of the bird, TraRsa cS. "iB^

THE MAMMALIAN EMBBYO

Fio. 106.

Embryo Rabbits of *bout Nine Days fbom thk liotUALtiu*-

{Frotu Kolliker.l

A. mBgnifieil 22 timea, and B. 21 times.

rjp. arua pelliioida ; r/ nicditlliiry groove ; A', medullary pint* if

tlie regiou of the fiit ure foro-brain ; h". medulkry plato iw

the region of the future luid-braiii ; i'^. fore-bmiii : a'l. "pti^

vesicle ; mh. mid-braiu ; hit. und h'". hind-braiu ; vv. i

blastic somite ; tU. vertebral zone ; pi. lat«ml zone ; Ax. *n^

A. heart ; ph. pericardial aectioin of body-cavity; no. ril«llii»*

<//.»

a fold.

X.] GENERAL DEVELOPMENT, 331

amnion (a^) are uow apparent both in front of and
behind the embryo.

The stmcture of the head and the formation of the
heart at thia age are ilhistrated in Fig. 107. The
widely open medullary groove [''/) is shewn in the
centre. Below it the hypoblast is thickened to form
the notochord dtf ; and at the sides are seen the two
tubes, which, on the folding-tn of the fore-gut, give rise
to the impaired heart*. Each of these ia formed of
an outer muscular tube of splanchnic raeaoblaat (ahk),
not quite closed towards the hypoblast, and an inner
epithelioid layer (Uih); and is placed in a special section
of the body cavity (ph), which afterwards forma the
pericardial cavity.

Before the ninth day ia completed great external
changes are usually effected. The medullary groove
becomes closed for its whole length with the exception
of a small posterior portion. The closure commences,
as in Birds, in the region of the mid-brain. Anteriorly
the folding-off of the embryo proceeds so far that the
head becomes quite free, and a considerable portion of
the throat, ending blindly in front, becomes established.
In the course of this folding the, at first mdely sepa-
rated, halves of the heart are brought together, coalesce
on the ventral side of the throat, and so give rise to a
median undivided heart. The fold at the tail end of
the embryo progresses considerably, and during its ad-
vance the allantois is formed in the same way as in
Birds. The somites increase in number to about twelve.
The amniotic folds nearly meet above the embryo.

> The details of tbe derelopmeat of the Leait ue described below
(eh. in.).

THE MAMMALIAN EMBRYO, [lIHAP.

Fla. 107.

Trahbveeihb Section throuqh the Head or a Babbit t

THE SAME AGE A8 FiG. 106 B. (From Kiilliker.)

B. ill a moru highly mugniiied representation of part of A.

^. medullary groove ; inp. medullary plate ; nc. medullary fold;

A. epiblast ; di. hypobloat ; c^. notochordal thickenjcg of

hypoblast ; ap. midivided meaobhist ; hp. somatic jnesoblust ;

] THE CRASUL flXXU^E. 3S3

d^ mhnrhnir meMbiMtt ; jA. [■»»»"i'-l seetko «f bo^T-
oThj ; oU. aaKakr vail oC hout ; OA. «gtaiaBoid kjw c(
heart ; ML btcnd oadiTided mnoUMt ; n*. fold a( hTpo-
bkst which will (onn tbe rmtnl wall ot tiu pbaryni. ; «^.

The later stagM in the derdopmeot proceed in tlie
main in die same manner as in the Bird The cranial
flexure soon becomes very marked, the mid-brain form-
ing the end of the long axis of the embryo (Fig. 108).
The sense organs have the usual development. Under
the fore-brain appears an epiblastic involution giving

D4Ta)'.

Adtabcgd Eubbyo op a Rabbit (ae
mil. mid-braiD ; fA. ttuiIamencepbaloQ ; ce. cerebral homiaphen
op. eye ; ie.v. fmirth ventricle ; nut. nioxiliary pmoeaa ; n»
maiiijibular nrch ; hif. hyoid aivh ; fl. fore-limb ; At hind-
limb ; um. uuibilical staJk.

' This Sgure was iiKwn by Mr Weldon.

334 THE MAMaiALlAM EMBBTO. [CBAF-

rise both to the mouth and to the pituitaiy body. Be-
hind the mouth are three well marked pairs of viscenl
arches. The tirst of these is the mandibular anh
(Fig, 108 md), which meets its fellow in the middle
line, and forms the postetior boundary of the mouth.
It sends forward on each side a superior maxillaij pro-
cess (nw) which partially forms the anterior marpn of
the mouth. Behind the mandibular arch are present i
well -developed hyoid (hi/) and a first branchial udi
(not shewn in Fig. lOS), There are four clefts, at in
the chick, but the fourth is not bounded behind by i
definite arch. Only the first of these clefts pereiflts u
the tympanic cavity and Eustachian tube.

At the time when the cranial fiexure appears, the
body also develops a sharp flexure immediately behind
the head, which is thus bent forwards upon the pos-
terior straight part of the body (Fig. 108). The amount
of this flexure varies somewhat in different forms. It
is very marked in the dog (Bischoff). At a later perio"!,
uiid in some species even before the stage figured, the
tail end of the body also becomes bent (Fig. 108), »
that the whole dorsal side assumes a convex curvatnre,
and the head and tail become closely approxtmat«d. Id
most cases the embryo, on the development of the tail,
assumes a more or less definite spiral curvature (Fig.
108), With the more complete devu-lopment of the
lower wall of the body the ventral tiexure partially (l*
appears, but remains more or less persistent till new
the close of intra-uterine life. The limbs are tbnuaJ »
simple buds in the same manner as in Birds. The buB
of the hind-limbs are directed somewhat forward^ i"
those of the fore-limb backwards.

THE HUMAN EMBRYO. 335

The homan embryo. Our knowk-dge as to the
early (ieveiopuiL-iit of the human embryo is in an un-
satistactory statt. The positive facta we know are com-
paratively few. aiid it is uot possible to construct from
them a histoiy of the developmunt which is capable of
satiafactoiy cumparison with that in other Ibrnis, unless
all the early embryos kuown are to be regarded as
abnornml. The moat remarkable feature in the develop-
ment, which was first clearly brought to light by Allen
Thomson in 1839, is tht; very early appearance of
branched villi. In the last few years several ova, even
younger than those described by Allen Thomson, have.
been met with, which exhibit this peculiarity.

The best preserved of these ova is one described by
Reichert'. This ovum, though probably not more than
thirteen days old, was completely enclosed by a decidua
reflexa. It had (Fig. 109 A and B) a flattened oval
fonu, measuring in its two diameters -i'5 mm. and
3'5 mm. The edge was covered with branched villi,
while in the centre of each of the tiattened surfaces
then:- was a spot free fiom villi. On the surface ad-
joining the uterine wall was a darker area (e) formed of
two layers of cells. Nothing certain has been made out
aliout the structure of ova of this age.

The villi, which at first leave the flattened poles
free, seem soon tu extend first over one of the flat sides
and finally over the whole ovum (Fig. 109 C).

Unless the two-layered region of Eeichert's ovum is
the embiyonic area, nothing which can clearly be

mtified as an embryo has been detected in these

Akntifie
H 1 Abb

AbbkndlnniKii der K6ni^. Akad. d. Win. jin Berlin, 1873.

THE MASIMALUN EMBEYU [l'HAI'.

Fio. 10;i.

The HmuH Ota onniKo earli staoes or oevBixieMissT.
(From Quain's Anatomt/.)

A. and B. Front and aide view ot an ovum figured by Reiohert,

supposed to be about thirteen days. e. embryonic iirea.
C. An ovum of about four or five weeks shewing the general

etructuroof the ovum before the formation of the placenta.

Part of the wail of the ovum is removed to show the embryo

in tilu. (After Allen Thomson.)

early ova. In an ovum described by Breus, aud in one
described long ago by WTiarton- Jones, a mass found in
tbe interior of the ovum may perhaps be interpreted
(His) as the remains of the yolk. It is, however, very
probable that all the early ova so far obtained are
more or less pathological.

The youngest ovum with a distinct embryo is one
described by His. Tliis ovum, which is diagrammati-
cally represtnted in Fig. Ill in longitudinal section,
had the form of an oval vesicle completely covered by
villi, being about S'5 mra. and 5'5 mm. in its two
diametei^, and flatter on one side than on tbe other.
An embryo witli a yolk-sac was attached to the inner
side of the flatter wall of the vesicle by a stalk, which
must be regarded as the allantoic stalk; the embryo

THE HUMAN £MBBTO. 337

FjG. 110.

Three Early HriiAH Eubryos. (Copied from His.)

A. Side view of an early embryo described by His.

E. Embryo of about 12 — 14 daja described by Allen Thom-
son.

C. TouDg embryo described by His.
am. amnion ; md. madullary groove ; oin. umbilical veaiclc ;

ch. chorion, to which the embryo is attached by a, stallc

and yolk-sac filled up but a very araall part of the
whole cavity of the vesicle.

The embryo, which was probably not quite normal
(Fig. 110 A), was very imperfectly developed; a me-
dullary plate was hardly indicated, and. though the
mesoblast was unsegmented, the head fold, separating
the embryo from the yolk-sac (um), was already in-
F. 4 B. 22

THE MAMMALIAN EMBBYO.

Fio. 111.

[chap.

I mc

DrAQRAUUATIC LOKGITDDINAL SECTION OF THE OVCII 10

WHICH THE Embryo (Fio. 110 a.) belosoed. (After Hi».)
am. muDiou; Nb. unibilioal.veaide.

dicated. The amnion (dm) was completely formed, and
vitelline vessels had made their appearance.

Two embryos descrihed by Allen Thomson are tul
slightly older than the above embrj'o of His. Botb of
them pnibably belong to the firat fortnight of preg-
iiant-y. In both cases the embryo was more or l«
folded off from the yolk-sac, and in one of them ll*'
medullary groove was still widely open, except in' to
region of the neck (Fig. 110 B). The allantoic atalfif
present, was not clearly made out, and the condidon of
the amnion was also not fully studied. The smaller ff
the two ova was just 6 mm, in its largest diameter, ami
was nearly completely covered with simple \-illi, di««
developed on one side than on the other.

In a somewhat later period, about the atage of >
phick at the end of the second day, the mednllaiy folil*

completely closed, the region of the brain alteodj

marked, and the cranial flexure commencing. Tbi!
mesoblast is divided up into numerous somites, and t^*
mandibular and first two biancliial arches ore inilicat«<i

THE HUMAN EJtBRYO.

339

The embryo is still but incompletely folded off from
the yolk-sac below.

In a still older stage the cranial flexure becomes
still more pronounced, placing the mid-brain at the end
of the long axis of the body. The body also begins to
be ventrally curved (Fig. 110 C).

Externally human embryos at thb age are charac-
terized by the small size of the anterior end of the
head.

The flexure goes on gradually increasing, and in the
third week of pregnancy in embryos of about 4 mm, the
limbs make their appearance.

The embryo at this stage (Fig. 112), which is about

Two VIEWS OP A Human Embryo of behs-ees the Thiru

AND FOUHTH WkeK.

A. Side view. (From KWliker ; after Allen Thoiuson.) o.

amuioD', 6. umbilical vesicle i o. mftDdibuUr arch; e. hyoid

arch;/, commencing anterior limb; g. primitive auditory

vesicle ; k. eye ; t. heart.
C. Dornal view to ahew the attachment of the dilated allantoic

Btalk to the chorion. (From a slcetob by Allen Thomson.)

am. amuion; aU. allantoitt ; j/». yolk-sac,

THE MAMMALIAN! EJOHITO, [CHAP.

equivalent to that of a chick on the fourth day, re-
sembles in almost every respect the uormal embryos of
the Amniota. The cranial Hexure ia as pronounced as
usual, and the cerebral region has now fully the normal
size. The whole body soon becomes flexed ventrally,
and also somewhat spirally. The yolk-sac (B ; ys) forms a
small spherical appendage with a long wide stalk, and
the embryo is attached by an allantoic stalk with a
slight swelling, probably indicating the presence of a
small hypoblaatic diverticulum, to the inner face of the
chorion.

A detailed history of the further development of
the human embryo does not fall within the province of

FlQUHES 8HEWCI0 THE EaRLY CBASGEB IN TBE fURM OF TUS

HuHAM Head, (Froni Qiiain's JnatOTn^.)

A. Head of an embryo of about four weeks. (After

Allen Thomson.)

B. Hood of iLD embryo of about six weeks. (After Eoker.)

C. Head of an embrjo of about nine weeks.

1. roandibular BToii ; 1'. persistent part of hyomandibular cleft ;
a. auditory veeicie.

|.] rXVERSION OF THE LAYERS. 341

this work ; while the later changes in the embryonic
membranea will be dealt with in the next chapter. For
the changes which take place on the formation of the
I fcce we may refer the reader to Fig. 113. ForafuUdis-
^OissioQ as to the relation between the human embryos
■just described and those of other Mnmmals, we refer the
■ leader to the Conip. Emhryolvgy, Vui. II. p. 224 et seq.
I The guinea pig, rat and moose present a pe-
I culiar metho<l of development, the details of which are
not tutirely understood, and we do not propose to
eiarame them here. Suffice it to say that the mode of
development gives rise to the so-called inversion of the
layers; so called because the outer layer of the em-
bryonic vesicle appeared to the older observers to be
funned of hypoblast and the embryonic epiblast to be
enclosed wilAi«.

CHAPTER XI.

EMBRYONIC MEMBRANES AXD YOLK-SAC.

In the Iifammalia. the early stages in the develc^
ment of the embryonic membranes are nearly the sanw
OS in Aves; but during the later stages the allantMi
enters into peculiar relations with the uterine w»iU,
and the two, together with the interposed portion rf
the subzonal membrane or false amnion (the nature of
which will be presently described), give rise to a vety
characteristic Mammalian organ — the placenta— uiW
the structure of which it mil be necessary to euM
at some length. The embryonic membranes vary »
considerably in the different forms that it will be ad-
vantageous to commence with a description of then
development in an ideal case.

We may commence with a blaatodermic vesicle closely
invested by the delicate remnant of the iwna radiata at
the stage in which the medullary groove is already
established. Around the embryonic area a layer of
inesoblast would have extended for a certain distance ;
so as to give rise to an area vasculosa, in which how-
ever the blood-vessels would not have become de&ult^

BAP. XI.] MEMBRANES OF RABBIT. .343

establtabecl. Such a vesicle is represented diagram-
tnatically in Fig. 114, i. Somewhat later the embryo
begins to be folded off first in front and then behinil
(Fig. Hi, 2). These folds result in a constriction sepa-
rating the embryo and the yolk-sac (ds), or as it is
called in Mammalian embryology, the umbilical vencle.
The splitting of the mesoblaat into a splanchnic and a
somatic layer has taken place, and at the front and
hind end of the embryo a fold {kg) of the somatic nieso-
blast and epiblast begins to rise up and grow over the
head and tail of the embryo. These two folds form the
commencement of the amnion. The head and tail folds

K^ the amnion are continued round the two sides of the

^Rmbiyo till they meet and unite into a continuous fold.

■ This fold grows gradually upwards, but before it has
completely enveloped the embryo the blood-vessels of
the area vasculosa become fully developed. They are
arranged in a manner not very different from that in

^^he chick.

H The following is a brief account of their arrange-

Haient in the rabbit : —

H^ The outer boundary of the area, which ia continually extend-
^ing further and further round the luubihcid veHio]i>, is marked by
. a venous ainua tenniuaUa (Fig. 1 14, «). The area ia uot, oa in
the chick, a nearly complete cu'cle, but is in front divided by a
deep indentation extending Inwards to the level of the heart. In
consequence of this indentation the sinus terminalis ends in
front in two branches, which bend inworda and fall directly inti)
the main vitoUiue vcitaa. Tho bl<x>d is brought from the dorsal
■orta by a series of lotend vitelline arteries, and not by a, aingle
pair Bs in the chick. These arteries break up into a more deeply
utnated arteriid network, from which tbe blood ia continued
{■art^iutothesinuii tonuinalis, and portly into a superficial venous

yil EJUJRYOKIC MEUBltANES AND YOLK-SAC. [CBiT.
Fio. 111.

XL] EMBRYONIC MEMBRANES.

Five Diagbammatic Figureb illustbatinq the Formatto.v
OF TSE Foetal Meubuanes uf a Maumal. (From K6lli-
kw.)

In 1, 2, 3, 4 the embrjo ia represented in longitudiaAl section.

1. Ovum with Eoiia pellucida, blastodermic vesicle, and
embryonic area.

2. Ovum vith commencing formation of umbilical vesicle

3. Ovum vith amnion about to close, and commencing
allantoia.

4. Ovum with viUoua subzonal membrane, larger allantois,
and mouth and anus.

5. Ovum in which the mosoblaRt of the allantois has ex-
tended round the inner surface of tbe aubzonal membrane and
united with it to form the chorion. The cavity of the allantois
is aborted. This fig. is a dic^ram of an early human uvum.

d. zona radiata ; d and «. processes of zona ; eh. aubzonal mem-
brane, outer fold of amniun, false amnion ; cA. chorion ; cA. i.
chorionic villi ; am. amnion; it. head-fold of amnion; ti. tail-
fold of amnion; a. epiblast ofembryo; o'. epiblastof non-em-
bryonioportof the blastodermic vesicle; fi. embryonic meao-
blast ; m', non-embryonic mesoblast ; df. area voficulosa ; si,
ainuH terminalia; «W, embryonic hypobList; i. non-embryo-
nic hyjK>blaat ; Jt/i. cavity of blastodermic vesicle, the greater
part of which becomes the cavity of urobilical vraicle ds. ;
ilff. stalk of umbilical vesicle; al. allantois; e. embryo; r.
space between chorion and amnion coiitatning albuminous
fluid ; vl. ventral boc^ wall ; kA. pericardial cavity.

3+6 EMBRYOSiC MEMBE4SES AND YOLK-SAC [CHIP.

network. The binder end of the heart is oontinued into tn
vitelline veins, each of which dividcH into an anterior And i
poHteriur branck The anterior branch is a limb of the smu
terminalia, and the posterior and emaller branch 'a lonSiined
towards the hind juirt of the sinua, nenr which it ends. On tit
way it reueives, on its outer aido, nunierons branches from tlie
vonoua network. The venoi;a network connects hy ila anaito-
moees, the poi>teriar branch of the vitelline vein and the aaw

Shortly after the establishment of the circulation <if
the yolk-sac the folds of the amnioa meet and ocwJesw
above the embryo {Fig. 114, 3 aDd4,aTn), After this the
inner or true amnion becomes severed from the out*^r
or false amnion, though the two sometimes remain ron-
uected by a narrow etalk. The apace between the tree
and false amnion is a continuation of the body caW)'.
The true amnion consists of a layer of epiblaslic B]ii-
theUum and generally also of somatic mesoblaat, wbiie
the false amnion consists as a rule of epiblast only;
though it is possible that in some cases (the rabbit ^1
the mesoblast may be continued along its inn^
face.

Before the two limbs of the amnion ate completely
severed the epiblast of the umbilical vesicle becoinea se[»"
rated from the subjacent mesoblaat and hypoblast of th«
vesicle (Fig. 114, 3), and, together with the false an
nion (sA) with which it is continuous, forma a comple'
lining for the inner face of the zona nuliata. The sp»*3<
between this membrane and the umbilical veaicie wii
tho attached embryo is obviously continuous with th*
body caidty {vide Figs. 114, 4 and 115). To this mei"-
brane Turner has given the appropriate name of »w-
xonal memhrane : by Von Baer it was called the »eiw»

XI.] ATTACHMENT OF THE OVUM. 347

envelope. It bood fuses with the zona radiata, or at
any rate the zona ceaaes to be distinguishable.

While the above changes have been taking place
the whole blastodermic vesicle, still enclosed in the
zona, has become attached to the walls of the uterus.
In the case of the typical uterus with two tubular
horns, the position of each embryo, when there are
several, is marked by a swelling in the walls of the
uterua, preparatory to the changes in the wall which
take place on the formation of the placenta. In the
region of each swelling the zona around the blasto-
dermic vesicle is closely embraced in a ring-like fashion
by the epithelium of the uterine wall. The whole
vesicle assumes an oval form, and it lies in the uterus
with its two ends free. The embryonic area is placed
close to the mesometric attachment of the uterus. In
many cases peculiar processes or villi grow out from
the ovum (Fig. 114, 4, se) which fit into the folds of
the uterine epithelium. The nature of these processes
requires further elucidation, but in some instances
they appear to proceed from the zona (rabbit) and in
other instances from the subzonal membrane (dog).
In any case the attachment between the blastodermic
vesicle and the uterine wall becomes so close at the
time when the body of the embryo is first formed out
of the embryonic area, that it is hardly possible to
separate them without laceration ; and at this period —
from the 8tli to the 9th day in the rabbit — it requires
the greatest care to remove the ovum from the uterus
without injury. It will be understood of course that
the attachment above described is at first purely super-
ficial and not vascular.

348 EMBHTOXIC MEMBRANES AND YOLK-SAC. [cHAP.

During the changes above described as taking place
in the amnion, the aliantoia grows out from the hind-
gut as a vesicle lined by hypoblast, but covered ex-
temally by a layer of splanchnic mesoblast (Fig. lli.J
and 4, ai)'. It soon becomes a flat sac, projecting into
the now largely developed space between the aubzonal
membrane and the amnion, on the dorsal side of ihe
embryo (Fig. 115, ALO). In some cases it estend^ w
as to cover the whole inner surface of the subzonal
membrane ; in other cases again ita extension is much
more limited. Its lumen may be retained or m»y be-
come nearly or wholly aborted. A fusion takes place
between the subzonal membrane and the adjoiuii^
niesoblastic wall of the aliantoia, and the two together
give rise to a secondary membrane round the onini
known as the chorion. Since however the allanW*
does not always come in contact with the whole inn^f
surface of the subzonal membrano the term chorion is
apt to be somewhat vague ; in the rabbit, for instance.
a considerable part of the so-called chorion is fonuw
by a fusion of the wall of tho yolk-sac with the sab-
zonal membrane (Fig. 116). The region of the choriiffl
which gives rise to the placenta, may in such cases ba
distinguished aa the true chorion from the reniainag
part which will be called the iialse chorion.

The mesoblast of the allantois, especially that p«*
of it which assists in forming the chorion, becomw
highly vascular ; the blood being brought to it by t«o
allantoic arteries continued from the terminal bifu^

I Tlia lij^ablnslio etemeot in tiio iiUantoiB U loiiMtuoM Tei7 oiidi
reduced, HO (bat theallautoiB majlnmiuiitf farmed otameolarU]*
Qf loeaablut.

MiMVAL. (From

THE Tartu, MEUBBAxxa or i
I Turner.)

Stiucturea which either are or have been at on earlier period
at iUveloiiroeot ooDtiououa witb each other are repreaented by
the saioe character of ahading.

lona witb villi ; u, sutttonal membrane ; £. epiUoit of
embryo ; am. amniiMi ; JC amniotic cavity ; if. o
of embtyo ; ff. hypoblast of embryo ; U V. umbilical »i
al. allautoiB; AW. allantoic cavity.

cation of the dorsal aorta, and retonied to the body
by one, or rarely two, allantoic veins, which join the
vitelline veins &om the yolk-sac From the onter sar-
face of the tnie chorion (Fig. 114, 5,cA,r, 116) villi grow
out and fit into crypts or depresaons which have in the

350 EMBRYONIC MEMBIUNES AND YOLK-SAC. [OUF.

BieaDtime made tlieir appearance in the walls of the
uterus'. The vllU of the chorion are covered by ui
epithelium derived from the subzonal niembiane, ami
are provided with a connective-tissue core conlfdning
an artery and vein and a capillary plexua coanectiDg
them. In most coses they assume a more or leas ar-
borescent form, and have a distribution on the s\irfm
of the chorion varying characteristically in differenl
species. The walls of the crypts into which the >illi
are fitted also become highly vascular, and a nutritive
fluid passes from the maternal vessels of the placenta
to the fcetal vessels by a process of diffusion ; while
there is probably also a siicretion by the epithelial
lining of the walls of the crypts, which becomes ab-
sorbed by the veasela of the foital villL The abow
maternal and foetal structures constitute together the
organ known as the placenta. The maternal portioD
conaiata essentially of the vascular crypts in th«
uterine walls, and the fcetal portion of more or leffl
arborescent vllU of the true chorion fitting into these
crypts.

While the placenta is being developed the folding
off of the embryo from the yoik-sac becomes more
complete; and the yolk-sac remains connected with tiiC
ileal region of the intestine by a narrow stalk, the vi-
telline duct (Fig. Ill, 4 and 5 and Fig. 115), consistiiig
of the same tissues as the yolk-sac, viz. hypoblast anJ
Bplanchnic mesoblaat. While the true splanchnic stalk

with tho opening* ol glu

They are believed b; Ereolnoi to b« fo

regeueralioD of the liniiig tisme ot Ibe irl

XL] THK YLXCESTA. 351

of the ydk-ne is becoming narrow, a somatic stalk
connacting the amnion with the waits of the embrj'o ia
also formed, and cloeely envelopes the Btalk both of the
•Uantois and the volk-sac. The somatic staJk together
with itit coDtenta ia known as the umbilicai cord. The
uesobloet of the eomatopleuric layer of the cord de-
velops into a kind of gelatiuoua tissue which cements
tc>gether the whole of the contents. The allantoic ar-
teries in tbe cord wind in a apiral manner round the
^Mntoic vein. The yolk-sac in many cases atrophies
completely before the close of intra-uterine life, but in
other cases it, like the otlier embryonic membranes, is
not removed till birth. The intra-embryonic portion of
the allantoic stalk gives rise to two structures, viz. to
(1) the urinary bladder formed by a dilatation of its
proximal extremity, and to (2) a cord known as the
urachns connecting the bkidder with the wall of the
body at the umbilicus, The urachus. in cases where
the cavity of the allantois persists till birth, remains as
an open passage connecting the intra- and extra-em-
bryonic parts of the allantois. In other cases it gradually
closes, and becomes nearly solid before birth, though a
delicate hut interrupted lumen would appear to persist
in it. It eventually gives rise to tbe ligamentum vesicte
medium.

At birth the fcetal membranes, inclu<iing the fcetal
portion of the placenta, are shed ; but in many forms
the interlocking of the ftetal villi with the uterine
crypts is so close that the uterine mucous membrane is
carrietl away with the fcetal part of the placenta. It
thus comes about that in some placenta the maternal
and fiftal parte simply separate from each other at birth,

EUBBYONIC U^MBRANES ANO TOLK-SAC. [CHAP.

and tbftt ia others the two remain iotiiuately locked
together, and both are ahed together as the after-birth.
These two forms of placenta are distinguished as non~
deciduate and deciduiUe, but no sharp line can be drawn
between the two types. Moreover, a laiger part of the
uterine mucous membrane than that actually entering
into the maternal part of the placenta is often shed in
the deciduate Mammalia, and in the non-deciduate
Mammalia it is probable that the mucous membrane
(not including vascular parts) of the maternal placenta
is either shed or absorbed.

Comparative history of the Mammaliatt fcetal
Tttembranes.

Two groups of Mammalia — the Monotremata and
the Mareupialia — are believed not to be provided with
a true placenta. Nothing is known of the arrangement
of the ftfital membranes in the former group of animals
(Monotremata). In the latter (Marsupialia) the yolk-
sac is large and vascular, and is, according to Owen,
attached to the subzonal membrane. The allautois on
the other hand is but small, and is not attached to the
subzonal membrane; it possesses however a vascular
supply.

Observations have hitherto been very limited with
regard to the fcetal membranes of this group of animals,
but it appears highly probable that both the yolk-sac
and the allantois receive nutriment from the walls of
the uterus-
All Mammalia other than the Monotremata and
Marsupialia have a true allantoic placenta. The pl».

XL] DISCOIDAL PLACENTA. 353

centa presents a great variety of forms, and we propose
first to treat the most important of these in succession,
jind then to give a general exposition of their mutual
affinities.

The discoidal placenta is found in the Rodeutia,
Insectivora, and Cheiroptera. The Rabbit may be
taken as an example of this type of placenta.

Tlie Babbit. In the pregnant female Rabbit severul ova are
generally fouud in eacb bora of the uterua. The general ooodi-
ticpa of tbefcetal-meaibranesattlie time of tbeir full development
is shown in Fig. 116.

The embryo ia eurrouaded by the amnion, nblcU is compaiO'
tivoly Biuail. The yolk-sac (ds) is large anJ attached to the
embryo by a long stalk. It has the form of a flattened sac
closely applied to about two-thirds of the Hurface of the subzonal
membrane. Tbe outer wall of this sac, adjoining the subzonal
membrane, is formed of hypoblast only j but the inner wall is
corered by the mesoblaat of the area vaMculosa, as indicated by
the thick black line ifd). The vascular area ia bordered by
the sinus terminalis (U). In an earlier stage of development the
jolk-sao had not the compressed form represented in the figure.
It is, however, remarkable that the vascular area nevur eitends
over the whole yolk-sac ; but the inner vaaoular wall of the yolk-
sac Aises with the outer wall, and with the subinnal membrane,
and so forms a false chorion, which receives ita blood supply
from the yolk-sac This part of the chorion does not develop
vascular villi.

The allantois (atj is a simple vascular sac with a large cavity.
Part of its wall is appUed to the subzonal membrane, and gives rise
to the true chorion from which there project numerous vascular
villi. These fit into uorresponding uterine crypta. It seems pro-
bsblu, from Biscboffs and KoUtker's olison'utions, that the sub-
zonal membrane in the area of the placenta beoomcs attached,
by means of villi, to the uterine wall even before its fusion with
the allantois. In the later periods of gestation the intermingling
of the maternal and fcetal ports of the pkcenta becomes very
F. 4 B. 23

354 ijibryonk; meubraA'es and volk-sac. [chap,

oloee, and the ploceota ia truly decijuate^ The cavity of the
allftntots persists till birth. Between the yolk-a&c, the ailantco^
aud the embryo, there ie left a large cavity filled with an albumi-
Dous Huid.

Fio. U6- -_

DuuHAMMATio LoNaiTiTDtiiAL SECTION OF A RABBir's Ovum
AT Aif Advanced Stage or Freonanct. (From Etilliker
after Bischoff.)

e. embryo ; a. aumion ; a. urachus ; al. allantois with Uood-
veaaels ; iA. aub-ional membrane ; pi. placental villi ; fiL
vaitcular layer of yolk-sac; ed. hypoblaatio layer of yolk-
sac ; ed'. inner portion of bypoblaat, and eti". outer portioa
of hypoblast lining the compressed cavity of the yo1k-«ao ;
dt. cavity of yolk-sac ; »t. sinus terminallaj r, space filled
with fluid between the amnion, the allantoia and the yolk-

The metadiscoidal type of placeuta is fouud in
Man and the Apea. The placenta of Man may be con-
veniently taken aa an example of this type.

XI,] METADISCOIDAL PLACENTA. 355

UuL The early atAgea iu the development of the fcetal
membranes in the human embrjo have oat been aatiafacbtnly
observed; but it ia kuowu that the ovum, shortly after its
«atrance into the uterus, becomes nttaubed to the uterine wall,
which iu the meantime boa undergone considerable preparatory
changes. A fold of the uterine wall appears to grovl round the
blastodermic Teaicle, and to form a complete capsule for it, but
the eioct mode of formation of this capsule ia a matter of infer-
ence and not of obeurvation. During the tirat fortnight of preg-
nancy viUi grow out, over the whole surface of tlie ovum. The
farther history of the early stages ia extremely obacure; what
ia known with reference to it will be found on p. 335 et seq. ; we
will here take up the history at about the fourth week.

At this Htuge a complete chorion has become formed, and is
probably derived from a growth of the meaoblaat of the alLantoia
(unaccompanied by the hypoblaat) round the whole inner surface
of the subzoual membrane. From the whole surface of the
chorion there project branched vaacubi.r proceaaes, covered by
an epithelium. The allaotois is without a cavity, but a liyim-
blastic epitheUum ia present iu the allantoic stalk, though
not forming a continuoua tube. The blood-vessela of the
chorion are derived from the usual allantoic arteries and vein.
The general condition of the embryo and of ita membranes at
this period is shewn diagrammatically in Fig. 114, 5, Around
the embryo is seen the amnion, already separated by a cousider-
able interval from the embryo. The yolk-aac ia ahewn at di.
Relatively to the other ports it is considerably amaller than
it waa at an earlier stage. The allantoic atalk is shewn at al.
Both it and the stalk of the yolk-sac are enveloped by the
amnion, am. The chorion with its vascular processes surrounds
the whole embryo.

It may be noted that the coudition of the choriim at this
stage is very similar to that of the nonnal diffused type of pla-
centa, described in the sequel.

While the above changes are tiikiug place in the embryonio

membranes, the blastodormia vesicle greatly increases in size, and

forms a cooaiderable projection from the upper wall of the

uterUB. Three regions of the uterine wall, in relation to the

23—2

356 EMBRYONIC MEMBItAfiES AND YOLK-SAC. [CHAP

blastodermic vesicle, ore usuiillj distiDgiiished ; and since tliv
Buperficial parts of bJI of these are tliroiTii off with the ofter-birtli,
each of thorn is called a decidua. The/ are represented »t I
somewhat later stage in Fig. 117. There is (1) the part of tbf
wall refieoted over the blautodermio yesicle, culled tbs dmdxn
r^fitza (dr) ; (2) the part of the wall forming the area idu»I
which the refleia is inserted, called the deeidua temtina (_di): (3
the general wall of the uterus, not related to the embrjo, oIIaI
the decidua vera (du).

The decidiiu reSexa and serotina together envelop the cborioii
(Fig. 114. 5), the prooessea of which fit into crjpta in thm
At this period both of them are highly and nearly uniforaJ)
vascular. The general cavity of the uterus is to a large ttM"
obliterated bj the ovum, but still perttieta as a apace filled viik
mucus, between the decidua reflexa and the decidua vera.

The changes which ensue from this period onwards are id;
known. The amnion continues to dilate (ita cavity being teMd.'
filled with amniotic fluid) till it comes very close to the ohorM
(Fig. 117, am); from which, however, it remains separated bj'
layer of gelatinous tissue. The villi of the chorion in thon^
covered by the decidua reflex.t, grodiully cease to be vaacnhr,
and partially atrophy, but in the region in contact with li"
deoiduA sDrotina increase and become more vascular and mon
arborescent (Fig. 1 17, t). The former region becomes known ••
the ohorion lieve, and the latter as the chorion frondofM. Tin
chorion frondosum, together with the decidua serotina, pves *"
to the placenta.

The umbilical vesicle (Fig. 117, nb), although it bewnn*
greatly reduced in size and flattened, persists in a reoogninW*
form till the time of birth.

Thedecidua refleia, by the disappearance of the vessels bll*
chorion Iteve, becomes non-vascu)ar. Ita tissue and that of ll*
decidua vera undergo changes which we do not propiwe **
descrihe here ; it idtimately fuses on the ooe hand with '^
ohorion, and on the other with the decidua vera. The oxa*'
brane resulting from its fusion with the latter structure
thinner and thinner as pregnancy advances, and is reduced I*
thin layer at the time of birUL

THE CHORION.
Pio. in.

IBaUiutic Secqdn op Pbsonaht Huiean ni&RDs wrrii

COKTAINBD Faros. (From Huilej after Longet)

tlbuitnic atolk: n^. umbiliusJ veHicle; am, auitiiou; rA.clio-

tiun ; dt. docidua eerotina ; c^u. tlecidu& vera ; dr. de<ii()iia

■ Rflexa ; I. fallopiau tube ; e. cervix utori ; u. uterus ; z. fccCtil

K *illi of true placenta; :f. villi or non-placental part of

V ohoiioiL

The placenta has a aomewbat discuidal tVinn, mtU a slightly
^*^vei uteriae surface and a. concave embrfoaiu surfkce. At ita
^S* it is continuoUB both with the deoidua reSexa and decidua
vera. Sear the centre of the embryonic surface is implaated tht
'Knbilical cord. As has already been mentioned, the placenta is '
^"Wed of the cleeidua aerotina and tlic^ ftcUl villi of the ohoriou
flmcloaom. The fcetal and matcniiil tiasues are far more closely
in the placenta of thu rabbit. The villi of the
I, which were originally comimrativoly siraple, become
id more complicated, and assume an extremely arboresceat
At bhth the whole placenta, together with tliu fkuied

sent J

EMBRYONIC MEMBUANKS AND YOLK-SAC. [CHAP.

cidua vera, and refloia, with which it is continuous, is shed ; and
the hluod-vessela thus ruptured are closed bj the contraction of
the uterine walls.

The metadiscoidal plotentii of Mnu and Apee and the diacoidal
pkeeuta of the Rabbit ore usually classified by auatomiats aa
diteoidal placentae, but it must be borne in mind that they differ
very widely.

In the Rabbit there is a dorsal placenta, which is co-citensive
with the area of coutnct between the alkntois and the subzonal
membrane, while the yolk-sac adheres to a large port of the
Bubzona] membrane. In Apes and Man the allontois spreads
over the whule inner surface of the subzonol membrane ; tha
placenta is on the central side of the embryo, and occupies only a
■mall part of the surface of the allantois.

Zonaiy placenta. Another form of dectduate pla-
centa is known as the zonary. This form of placenta
occupies a broad zone of tho chorion, leaving the two
poles free. It is found in the Carnivora, Hyiax, Elephas,
and Orycteropua.

In the Dog, which may be takeo as a type, there is a lai^a
Tascular yolk-sao formed in the usual way, which does not how-
ever fuse with the chorion. It has at lirGt on oval shape, and
persists till birth. The allantois first grows out on the dorsal
side of the embryo, where it coalesces with the subzonal mem-
brane, over a small disooidal area, and there U thut formed a
TVfdintentary dUcoidal placenta cloHely resembling tiint of the
Rabbit.

The area of adhesion between the out«r part of the allftntois
and subzonal nieinbrane gradually spreads over the whole inte-
rior of the subzonal membrane, and vascular vilU are formed orer
the whole area of adhesion except at the two extreme poles of the

With the full groivth of the allantois there is formed a broad
fdacental zone, with numerous branched villi fitting into corre-
apoading pita which ate not trvie glands but special develop*

XI.] NON-DECIDUATE PLACENTA. 359

menta of the uterine surface. The malcriiul and fcctiU structurt<a
become closely interlocked and highly viiscular ; And at birth a
Lirge part of the maternal part ia carried awny with tl^o placenta j
some of it kowever still remains attached to the mtiscular wall of
the uterus. The zone of the placenta diminishes greatly iu pro-
portion to the chorion as the latter elongates, and at the Aill
time the breadth of the zone is not more than about one-fifth of
the whole length of the chorion.

At tlie edge of the placental zone there is a very small |«rtion
of the uterine muooua membrane reflected over the non-placantfll
part of the chorion, so as to form a small reflexa analogous with
the reflexa in Man.

The most important of the remaioing types of pla-
centa are the diffuse and the polycotyledonaiy, and
these placentiR are for the most part non-deciduate. In
the diffuse plajjouta, found iu the Horse, Pig, Le-
murs, etc., the allantoia completely envelopes the em-
bryo, and villi are formed on all parts of the chorion,
excepting over a small area at the two poles.

Id the polycotyledonaiy placenta, which is charac-
teristic of the Ruminantia, the allantois grows round the
whole inner surface of the subzonal membrane ; the
placental villi are however not imiformly distril»uted,
but collected into patches or cotyledons, which form as
it were so many small placentaj. The fcetat villi of
these patches fit into corresponding pits in thickened
patches of the wall of the uterus.

Comparative histology of the Placenta.

It does not fall within the province of this work to
treat from a histological standpoint the changes which
take place in the uterine walls during pregnancy. It
will, however, be convenient to place before the reader

360 EMBRYONIC MEMBRANES AND TOLK-SAC. [CttlF.

a short statement of the relations between the materad
and fretal tissues in the different varieties of placenta.

The Bimplest known condition of the placenta ia
that found in the pig (Fig. 118 II.). The papilk-likc
foBtal villi fit into the maternal crypts. The villi (f) u*
formed of a connective tissue core with capillaries^ an^
are covered by a layer of very flat epithelium («) df-
rived from the subzonal membrane. The matfinul
ciypts are lined by the uterine epithelium {«'), inune-
diately below which is a capillary plexus. The maternal
and fcetal vessels are here separated by a double epi-
thelial layer. The same general arrangement holds
good in the diffused placentie of other forms, and b the
polycotyledonary placenta of the Riiminantis, but the
fcetal villi in the latter (III.) acquire an arborescent fonn.
The maternal vessels retain the form of capillaries.

In the deciduate placenta a much more compli-
cated arrangement is usually found. In the typics!
zonary placenta of the fox and cat (IT. and V.), the
maternal tissue 'm broken up into a complete trabecular
meshwork, and in the interior of the trabecule there
run dilated maternal capillaries (d'). The trabeculB
are covered by a more or less columnar uterine ep-
thelium (e'), and are in contact on every side with fatal
villi. The capillaries of the fcetal villi preserve thar
normal size, and the villi are covered by a flat epitlieliil
layer (e).

In the Sloth (VI.) which has a discoidal placenta ths
maternal capillaries become still more dilated, and tlie
epithelium covering them ia formed of very flat poly-
gonal cells.

HISTOLOGY OF THE PLACENTA.

364 EMBRYONIC MEMBRANES AND YOLK-SAC. [c HAP. XL

Evolution of tlie placenta. Excluding the mu-

supials whose placentation is uot really known, tie
;u"rangeineiit of the fcetal membranes of the Babbit is
the most primitive observed. In this type the aiiaato)
and yolk-sac both function in obtaining uutrimeiH
from the mother ; and the former occupies only & fiaull
tliscoidal area of the subzonal membrane. In all higher
typea Uie allantois gradually spreads out over the whole
inner surface of the subzonal membrane and its im-
portance increases ; while that of the yolk-sac as a nu-
tritive organ decreases. In the diffuse type of placentJ
simple villi are present over nearly the whole suriiice of
the chorion. In the remaining types the villi hfcome
more complicated and restricted to a smaller siw
(raeta-diacoidal, zonary, &c,) of the chorion ; though in
the early stages they are more scattered and Bimpkr.
in some cases occupying nearly the whole surface of tin'
chorion. It therefore seems probable that the placait*
of Man has been derived not directly from the discoiiW
placenta of the Rabbit, but from the diffuse plareot)
such as is seen in the Lemurs, etc., and that geneitiif
the zonary, cotyledonary, &c. types of placenta la'
been derived from the diffuse by a concentration aw"
L increase in the complexity of the foetal villi.

CHAPTER XII.

THE DEVELOPMENT OF THE ORGANS IN BIAMMALIA.

In chap. X. we have described the early stages and
general development of the mammalian embryo. In
the present chapter we propose to examine the for-
mation of such mammalian organs as differ in their
development from those of the chick. This will not be
a work of any considerable extent, as in all essential
points the development of the organs in the two groups
is the same. They will be classified according to the
germinal layers from which they originate.

The organs derived from the epiblast.

Hairs are formed in solid processes of the deep
(Malpighian) layer of the epidermis, which project into
the subjacent dermis. The hair itself arises firom a
comification of the cells of the axis of one of the above
processes ; and is invested by a sheath similarly formed
from the more superficial epidermic cells. A small
papilla of the dermis grows into the inner end of the
epidermic process when the hair is first formed. The

366 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.

first trace of the hair appears close to this papilla, but
aoon increases in length, aad when the end of the hair
projects from the surface, the original Bolid process of
the epideimia becomes converted into an open pit, the
lumen of which is filled by the root of the hair.

The development of nails has been already described
on p. 283.

Glands. The secretory part of the various glandular
atnictures belonging to the skin is invariably formed
from the epidermis. In Mammalia it appears that
these glands are always formed as solid ingrowths of the
Malpighian layer. The euds of these ingrowths dilate
to form the true glandular part of the organs, while the
stalks connecting the glandular portions with the sur-
face form the ducts. In the case of the sweat-glands
the lumeu of the duct becomes first established ; its
formation is inaugurated by the appearance of the
cuticle, and appears first at the inner end of the duct
and thence extends outwards. In the sebaceous glands
the first secretion is formed by a fatty modification of
the whole of the central ceils of the gland.

The muscular layer of the secreting part of the
sweat-glands is said to be formed fiYini a modification oS
the deeper layer of the epidermic cells.

The mammary glands arise in essentially the same
manner as the other glands of the skin. The glands of
each side arc formed as a solid bud of the Malpigbian
layer of the epidermis. From this bud processes sprout
out, each of which gives rise to one of the numerous
glands of which the whole organ is formed.

Xll.] TUK HIND BRAIN,

The central nei-votis system.

The development of the spinal cord in Mammals
differs in no important respects from that of the chick,
and we have nothing to add to the account we have
already given of its general development and histoge-
nesis in that animal. The development of the brain
however will be described at greater length, and some
additional facts relative to the development of the
Avian brain will be mentioned.

The first differentiation of the brain takes place in
Mammalia before the closure of tfie medullary folds,
and results as in the chick in the formation of tlie three
cerebral vesicles, the fore-, mid- and hind-brain {Fig.
106, B). A cranial flexure precisely resembling that of
the chick soon makes its appearance.

The hind brain early becomes divided into two
regions, the rudimentary medulla oblongata and cere-
bellum.

The posterior section, the medulla, undergoes changea
of a somewhat complicated character. In the first place
its roof becomes very much extended and thinned
out. At the raphe, where the two lateral halves
of the brain originally united, a separation, as it were,
takes place, and the two sides of the brain become
pushed apart, remaining united by only a veiy thin
layer of nervous matter, consisting of a single row of
flattened cells (Fig. 40). As a result of this peculiar
growth in the brain, the roots of the nerves of the two
sides, wliich were originally in contact at the dorsal
summit of the brain, become carried away from one
another, and appear to rise at thf sides of the brain.

368 DEVELOPMENT OF ORGAKS IN HASIMALIA. [CBU'

The thin roof of the fourth ventricle thus fonud
is somewhat rhomboidal in shape.

At a later period the blood-vessels of the pi
mater form a rich plexus over the anterior part of
this thiu roof which becomes at the same time some-
what folded. The whole structure is knowa aa titi
tela vasculosa or choroid plexus of the fourth wirirttif
(Fig. 119, chd 4). The floor of the whole hind-tap
becomes thickened, and there very soon appears on ili
outer surface a layer of longitudinal oon-tnedullateil
nerve-fibres, similar to those which first appear on the
spinal cord (p. 252); Tht^y are continuous with a giinilu
layer of fibres on the floor of the mid-brain, where
they constitute the crura cerebri. On the veDtral floM
of the fourth ventricle is a shallow continuatioa of the
anterior fissure of the spinal cord.

Subsequently to the longitudiaal fibres alreadjr qnkni ^^
there develope first the olivary bodiea of the ventntl aide rf tb
medullfl, and at a still later [leriod the pyramida. The fiacwl'
teretes iu tho cavity of the fourth vsutricle are developed abortl!
before the pyramida.

When the hind-brain becomes divided into tm
regions the roof of the anterior part does not becoiW
thinned out like that of the posterior, but on the con-
trary, becomes somewhat thickened and forms a baol'
like structure roofing over the anterior part of tlw'
fourth ventricle (Fig. 39 ch).

This is a rudiment of the cerebellum, and b W
Craniate Vertebrates it at first presents this ss^
structure and insignificant size.

In Birds the cerebellum attains a very considenble
development (Fig. 119 c62), consisting of a folded

THE HIND-BKAIN. 3C!f

with an arbor vitas, into wLich the fourth ventricle
longed. There sire two gmatl lateral lobes, ap-
Jy equivalent to the flocculL
Eb Mammalia the cerebellum attuns a still greater
Blopmeat The median lobe or vermiform process

"BGiTCBiMAi. Secnos THBoroa the Brain or a Chick op
Ten Days. (After Mihalkovics.)

t. cerebral hemispheres ; alf. uifactorf lobe ; alj\, ol&ctoiy
nerve ; ggt. corpus striatum ; oma, anterior commisfiiire ;
oU 3. choroid plexus of the third ventricla ; pin. pineal
glanil ; anp. posterior commissura ; trvi. lamina tenninaliB ;
cAm. optic chiasma ; iit/ iuTimdibuIiini ; hpk. pituitary bodj ;
b^. commissure of Sylvius [roof of iter a tertio ad quarti
ventricuUuu) ; vma. velum medulla anteriua (valve of Tieuii-
aens) ; fbl. cerebellum ; eM 4. eboroid plexus of the
ventriclo; obi i. roof of fourth rentricle ; uhl. medulla
gata ; ptii. coDimiHsural part of medulU ; mv. aheatit
braiu ; hh. basilar arterj ; crU. intemal carotid.
P. « B. "SA

fourth ^^^I
obloDt^^^H

370 DEVELOPMENT OF ORGANS IS MAMMALIA- [ClUr,

IB first developed. In the higher Mammalia the lateral
parts constituting the hemispheres of the cerebellum
become formed as swellings at the mdes at a couaiiler-
ably later period ; these are hardly developed in the
Monotremata and Maraupialia.

The cerebellum is cormected with the roof of the mid'btun in
front imd with the choroid pleiua of the fourth veDtricIs beUml
by delicate membnuious atmctures, known as the mliim OK-
dullro oiiterius (vulve of Vieusaens) (Fig. 1 19 t-ma) and the nluB
inedullic ]H)8teriua.

Tiie poDs Varolii is formed oa the ventt.il stdu of the Ituw "I
the cerebellar r^ou as a. bundle of transverse fihra at oWttb
noma time as the olivary bodies. It ia represented in Birdi ti;
a small number of transverse fibres on the Boor of the hiad-bniii
imDiediately below the uerebellimi.

The mid-brain. The changes undergone by the
mid-brain are simpler than those of any other part of
the brain. It forms, on the appearance of the craniii
flexure, an unpaired vesicle with a vaulted roof lUnl
curved floor, at the front end of the long axis of the
body (Fig. 67. MB). It is at this period in Uommali*
as well as in Aves relatively much larger than in iha
aduit: its cavity is known as the iter a tertio ii
quartum ventriculwm or aqueductus Sylvii.

The roof of the mid-brain is shitrply CDnstriott^
off from the divisions of the brain in front of iwi
behind it, but these constrictions do not extend to tht
floor.

In Mammalia tlje roof and sides give rise to
pairs of prominences, the corpora quadrigomina.

These prominences, which are simply thickeninp
not containing any prolongations of the iter, beconw

THE FOKE-BRAIS.

371

V- first \-isibie on the appearance of an obiiqae transverse
B- fttrrow, by which the whole mid-brain is divided into an
W anterior and posterior portion. The anterior portion is
farther divided by a lon^tudinal furrow into the two
anterior tubercles (nates) ; but it is not until later on
that the posterior portion ia similarly divided longitu-
dinally into the two posterior tubercles (testes).

The floor of the mid-brain, bounded posteriorly by
the pons Varolii, becomes developed and thickened into
the crura cerebri. The corpora geniculata interna also
belong to this division of the brain.

Fore-bram, The early development of the fore-

Imtin in Mammals is the same as in the chick. It forms

at first a single vesicle without a trace of separate

I divisions, but very early buds ofl" the optic vesicles,

! whose history is described with that of the eye. The

anterior part becomes prolonged and at the- same time

' aomewhat dilated. At first there is no sharp boundaxy

between the primitive tbie-brain and its anterior

prolongation, but there shortly appears a constriction

i which passes from above obliquely forwards and down-

[ muds.

Of these two divisions the posterior becomes the

thalamencephalon, while the anterior and larger division

I forms the rudiment of the cerebral hemispheres (Fig.

" 9 cer) and olfactory lobes. For a considerable period

"s rudiment remains perfectly simple, and exhibits no

[na, either externally or internally, of a longitudinal

MDStriction dividing it into two lobes.

The thalammcephalou forms at first a simple

vesicle, the walls of which are of a nearly uniform tliick-

B and formed of the usual spindle-shaped cells.

372 DEVBLOFMEKT OK OHGANS IN MAMMAUA. [CHil'.

The cavity it contains is known as the third ventriek
Anteriorly it opens widely into the cerebral rudinn^l,
and posteriorly into the ventricle of the roid-braiii.
The opening into the cerebral rudiment becomes the
foramen of Monro.

For convenience of description we may divide the
thalamencephalon into three regions, viz. (1) the floor,
(2) the sides, and (3) the roof.

The floor becomes divided into two parts: an an-
terior part, giving origin to the optic nerves, in wbJcb is
formed the optic chiasma ; and a posterior part, vhicli
becomes produced into a prominence at first incon-
spicuous— the rudiment of theinftindibulum (Fig. 39 /ni.
This comes in contact with the involution from llw
mouth which gives rise to the pituitaiy body (Fig.
39 pt).

In Birds, although there is a close counecUoo be-
tween the pituitary body and the infiindibulum, therp
is no actual fusion of the two. Id Mammalia the caw
is different. The part of the infundibulum which lies
at the hinder end of the pituitary body is at first •
simple finger-like process of the brain (Fig. 120 in/ii
but its end becomes swollen, and the lumen in this
part becomes obliterated. Its cells, originally similu »
those of the other parts of the nervous system, and efen
containing differentiated nerve-fibres, partly atrophy
and partly assume an indifferent form, while at thp
same time there grow in amongst them nuineroa*
vascular and connective -tissue elements. The procw*
of the infundibulum thus metamorphosed becomes in-
separably connected with the true pituitary body, <^
which it is usually described as the posterior lobe.

THE TUALAMENCEPHALON.

37a

Llhei

In the later st^es of development tlie unclianged
portioD of the mlundibulum becomes gradually pro-
longed and ibrms an elongated diverticiiUim of the
third ventricle, the apex of which is in contact with
the pituitary body (Fig. 120 kph).

The poBterior part of the primitive iurimdibuliiu becomeB the
corpus albicans, which ia double io Man and the higher Apes ;
the ventral part of the posterior wall forms the tuber ciuereum.
IjiteraDj-, at the junctioD of the optic thalami and iafundibulum,
there are continued aoine of the fibrea of the cnini cerebri, which
probably deriTed from the walls of the infimdibulam.

The sides of the thalamencephalon become very
early thickened to form the optic thalami, which con-
atitate the most important section of the thalamen-
cephalon. These are separated on their inner aspect
from the infundibular region by a somewhat S-shaped
groove, known as the sulcus of Monro, which ends in
the foramen of Monro. They also become secondarily
united by a transverse commissure, tho grey or middle
commissure, which passes across the cavity of the third
ventricle.

The roof undergoes more complicated changes. It

.becomes divided, on the appearance of the pineal gland

a small papilliform outgrowth (the development of

'hich is dealt with below), into two regions — a longer

iterior in front of tho pineal gland, and a shorter pos-

ior. The anterior region becomes at an early period

Lceasively thin, and at a later period, when the roof oi'

thalamencephalon is shortened by the approach of

^e cerebral hemispheres to the mid-brain, it becomes

(vids Fig. 120 did 3) considerably foMed, while at the

a vascular pitxua ts formed in the pis q

37i DEVELOPMENT OF OBOANS Di HAUUiXIA. [CUif. ,

^ .V

LoNQiTUDiNAL Vebtical Seotion theodgh the Astebior
Part ov the Bsain of ah Eubrtu Babbit of focb
CENTmEiREa. (After Aliholkovics.)

Tlie section passes through the median line so that tha oetc-
bral hemispheres are not cut ; their poeitioQ is however indkaM
in outline.

gpt, septum lucidum formed bj the oooleacence of the inner w»U»
of pnH of the cerebral hemispheres ; cma. anterior wn-
misaure ; frx. vertical pillars of the fornix ; eal. genu of
corpus oalloeum ; trm. lamina terminalis ; hmt. cerekil
temispherea ; olf. olfactory lobes ; ad. artary of oorpaa
calloBum ; Jinr. position of foramen of Monru ; chd 3. choraiJ
plesua of third ventricle; pm. pineal gland ; emp. postenw
oommissura ; biftn. lamina uniting the lobes of tba DUa-
brain ; cAm. optiu chiaama ; /iph. pituitary body ; inf. infuo-
dibulum ; pTu. pons Varolii ; pde. cerebral peduncIOH ; agd^
iter a tertio ad ijuartum Tentriculum-

Xn,] THE PINEAL GLASD. 375

above it. Ou the accomplishment of these changes it
is known as the tela choroidea of the third veutricle.

In the roof of the third ventricle behind the pineal
^land there appear transverse comniissural fibres, form-
ing a structure knov,-n as the posterior commissure,
which connects together the two optic thalami.

The most remarkable organ in the roof of the thala-
mencephalon b the pineal gland, which is developed as
a hollow papilliforoi outgrowth of the roof, and is at
first composed of cells similar to those of the other
parts of the central nervous system (Fig. 120 pin). It
is directed backwards over the hinder portion of tlie
. roof of the thaiameQcepholon.

In Birds (p. 116) the primitive outgrowth to form
liiflie pineal gland becomes deeply indented by vascular
tive-tissue ingrowths, so that it assumes a den-
ritic structure (Fig. lid pin). The proximal extremity
tached to the roof of the thalamencephalon soon
I solid and forms a special section, known as
; infra-pineal process. The central lumen of the
e part of the gland finally atrophies, but the branches
1 remain hollow. The infra-piueal process becomes
xluced to a narrow stalk, connecting the braiiclied
■tion of the body with the brain.
In Mammalia the development of the pineal gland
is generally similar to that of Birds. The original out-
growth becomes branched, but the follicles or lobes lo
which the branching gives rise eventually become solid
(Fig. 120 pin). An infra-pineal process is developed
comparatively late, and is not sharply separated from
the roof of the brain.

No satiafactoiy suggestions have yet been offered aa

376 DEVEL(.rPMENT OF OHGAKS IN BUHMALIA. [l'HAP.

to the nature of the pineal gland. It appeara to possesa
in all forms an cipithelia! structure, but, except at the
baso of the stalk (infra-pineal process) in Mammalia, in
the waU of which there are nerve-fibres, no nerviKu
structures are present in it in the adult state.

Ths oerebral hemispheres. It will be conveaient
to treat separately the development of the cerebral
hemispheres proper, and that of the olfactory lobes.

In the CL'rebral rudiment two parts may be dis-
tinguished, viz, the floor and the roof. The former pTa
lise to the ganglia at the base of the hemispheres, the
corpora atriata, the latter to the hemispheres proper.

The first change which takes place consists in lie
roof growing out into two lobes, between which a shallow
median constriction makes ita appearance (Fig. 121V

DiAGRutuATir LoNGiToniBAL Horizontal Section throuqh

THE FORK-BBAIH.

3.11. third veutricle ; Iv, lateral Tentricle ; 'f. l&miuiL UrmiMJii;
e. cerebral hemlaphero ; opJJi. optic thalamus.

THE CEREBRAL HEMISPHEHES.

a77 I

The two lobea thus formed are the rudiments of the
two hemispheroa. The cavity of eacli of them opens
by a widish aperture into a cavity at the base of the
cerebral rudiment, which again opens directly into the
cavity of the third ventricle (3 v). The Y-shaped aper-
ture thus formed, which leads from the cerebral hemi-
spheres into the third ventricle, is the foramen of
Monro. The cavity (Iv) in each of the rudimentary
hemispheres is a lateral ventricle. The part of the
eerebnun which lies between the two hemispheres, and
passes forwards from the roof of the third ventricle
round the end of the brain to the optic chiasma below,
is the rudiment of the lamina terminalis (Figs. 121 It
and 123 trm). Up to this point the development of
the cerebrum is similar in all Vertebrata, and in some
forms it practically does not proceed much fiirther.

The cerebral hemispheres undei^o in Mammalia the
most complicated development. The primitive un-
paired cerebral rudiment becomes, as in lower Ver-
tebrates, bilobed, and at the same time divided by the
ingrowth of a septum of connective tissue into two
^distinct hemispheres (Figs. 125 and 121/ and 122 i).
iFrom this septum is formed the falx cerebri and other

The hemispheres contain at first very large cavities.
fiBommunicating by a wide foramen of Monro with the
Quid ventricle (Fig. 124). They grow rapidly in size,
and extend, especially backwards, and gradually cover
the thalamencephalon and the mid-brain (Fig. 122 I,/).
The foramen of Monro becomes very much narrowed
and reduced to a mere slit.

The 'walls are at firet nearly uniformly

378 DEVELOPMENT OF ORGANS IN UAHUALU [CHAP.
Fill. 139.

Bhain of a Thrke Months' Human Eubrio : HATOBALint

(From Kiilliker.)
I. From above with the dorsal {Utrt nf hemispkerea and mjd-

bruin removeJ ; 2. From below. / anterior part of not i«»ll

of the hemiajihere ; /'. oornu ammonia ; tho. o|itic UioUuiiu ;

•■it. corpus striatum ; to. optic tract ; em, corpora mtmni!-

laria ; p. pons Varolii.

the floor becomes thickeued ou each aiile, and fpv« ti«
to the corpus striatum (Figs. 124 and 125 «(). The
corpus striatum projects upwards into each lateral 'eo-
tricle, and gives to this a somewhat semilunar form, the
two horns of which constitute the permanent anleriur
and descending coraua of the lateral ventricles (Fig. 126

St).

With the further growth of the hemisphere the eax-
pu3 striatum loses its primitive relations to the de-
scending comu. The reduction in size of the foramea
of Monro above mentioned is, to a large extent, causei)
by the growth of the corpora striata.

The corpora striata are united at their posteno'
border with the optic thalami. In the later stages of
development the area of contact between thoeo tw
pairs of ganglia increases to a large extent (Fig. 1SG)>

THE COKPORA STRIATA.

S79

' xil]

Bud the boundary between them becomes somewhat
obscure, so that the sharp distinction which exists
the embryo between the thalamencephalon and
I cerehrd hemispheres becomes lost.

iwx Section through tm Brais of a Rahbit ok
Five Centiketeeb. (After Mihalkovics.)

The seotion passes through near!j the poatorior bonier of thu
gptam lucitJum, immediatuly in front of the foramen of Monro.

; cerebral hemispherea ; eal. cor]>ii9 coUosum ; amm. comu
mis (IiipjxMampuB major) ; cm». superior commissure
of the comua ammonis ; tpt. septum luciilum ; frx 2. anterior
]nUAra of the fornix ; cnta. anterior commissure; trm.. lominn
tenninoliB ; tir. corpus ntriatum ; l^, nuclena lenticuloris
of corpus striatum; vtr 1, kteral ventricle; vlr 3. thinl
ViBtaiele t ipL aUt between oerebral hendaphena.

380 DEVELOPMENT OF ORGANS IN MAMMALIA. [OHAf,

The outer wall of the hemispheres gradually Uuck-
ena. while the inner wall becomes thinner. In the
latt«r, two curved folds, projecting towards the inteiia
of the lateral ventricle, become formed. These foldf
extend from the foramen of Monro along nearly tiw
whole of what afterwards becomes the descending coniu
of the lateral ventricle. The upper fold becomes tJie
hippocampus major (comu ammoms) (Figs. 123 atnm,
124 and 125 A, and 126 am).

The wall of the lower fold becomes very tliin, anda
vascular plexus, derived from the connective-tiasae
i^cptum between the henriispheres, and dmilar to thatof
the roof of the third ventricle, is formed out^de it. It
constitutes a. fold projecting into the cavity of the
lateral ventricle, and together with the vascular con-
nective tissue in it gives rise to the choroid plexus uf
the lateral ventricle (Figs. 124 and 125 pi).

It is clear &om the above description that a marginal
fissure leading into the cavity of the lateral ventricle
does not exist in the sense often implied in works oq
human anatomy, since the epithelium covering the
choroid plexus, and forming the true wall of the brain,
is a continuous membrane. The epithelium of the
choroid plexus of the lateral ventricle is quite inde-
pendent of that of the choroid plexus of the thiri
ventricle, though at the foramen of Monro the roof dl
the third ventricle is of course continuous with ihi
inner wall of the lateral ventricle (Fig. 124 s). Th*
i-asciila^- elements of the two plexuses form however i
I'ontinuous structure.

The must characteristic parts of the
cerebrum are the commissures connecting the tn(

xn.J

Jietnisplieres. These commissures ai-e (1) the anterior
(2) tlie fornix, and (3) the corpus callosum,
two latter being peculiar to Mammalia.

THE CEREBKAI, COMMISSUBKS.

381

iTxasE Seotion throitqh the Bbaik or a Sheep's
EUBRYO or 27 CM. IN LKNOTH. (From KOlliker.)
Tbo section passes through the level of the foramen i.<r

corpus etriatuni ; m. fnramen of Monro ; t. third ventridc ;
pi, choroid plexus of lateral ventricle ; f. fnti cerebri ; tli.
anterior piirt of optic tbalomuB ; ch. upt[c chiasma ; o. optio
nerve ; c fibres of the cerebral peduncles ; A. oomu ani-
monis : p. pborTm ; »a. pre-epbenoid bone; a. ortii't"-
ephenoid bone ; ». pointa to part of the roof of the brain at
the junction between the roof of the third ventricle anil
the lamina termioalis ; I. lateral venttiole.

3S2 DEVELOPMENT OF ORGANS IN MAUMALIA. [CHAP.

By the fusion of the inner walls of the hemispheres
in front of the lamina terminalis a solid septum is
formed, continuous behind with the lamina tenninalis.

TraNBVBHBE BSQtlOH THBOUBH TBK BraIM OF A SbeeP«

Embryo of S-7 cm. ik lehoth. (From KQlliker.)
The HectioD is taken a short distance behind the aeetion

represented in Fig. 1S4, and posses through the poaterior part of

the hemisphures and the third ventricle.

tl. corpus striatum ; lA. optic thalamus ; to, oiitic tract ; r. third
ventricle ; if. roof of third ventricle ; r. fibrea of c«rebral
pedunclea ; <•'. divergence of these fibres into the walls of the
hemiepherea ; i: Interal ventricle with choroid plexus pi ■
A. comu ammonis ; /. primitive fall ; am. alisphenoid ; a.
orbito-aphenoid i *a, praapbenoid ; p. pharynx ; itJt. Meckel's
cartilage.

tt.] THE CORPUS CAILOSUM. 383

and below with the corpora striata (Figs. 120 and 12S spt).

It is by a series of differentiations within this septum,
the greater i>art of wliich gives rise to the septum luci-
dum, that the above comniisaiires originate. In Man
there is a closed cavity left in the septum known as the
6fth ventricle, wliich has however no communication
with the true ventricles of the brain.

In this septum there become first formed, below and
behind, the transverse fibres of the anterior commissure
(Fig. 120 and Fig. 123 cma), while above and behind
these the vertical fibres of the fornix are developed
(Fig. 120 and Fig. IZS/rx 2). The vertical fibres meet
above the foramen of Monro, and thence diverge back-
wards, aa the posterior pillars, to lose themselves in the
comu ammonis (Fig. 123 amm). Ventrally they are
continued, as the descending or anterior pillars of the
fornix, into the corpus albicans, and thence into the
optic thalami*.

The corpus callosum is not formed till after the
anterior commissure and fornix. It arisea in the upper
port, of the septum formed by the fusion of the lateral
walls of the hemispheres (Figs. 120 and 123 cal), and
at first only its curved anterior portion — the genu oi
rostrum — is developed. This portion is alone found
in Monotremes and Marsupials, The posterior portion,
which is present in nil the Monodelphia, is gradually
med as the hemispheres are prolonged further hack-

■ Heceut oliaotvations tend to ebow that the ulterior pillurs of the

romii eai iu the corpus aibiouia; and tbnt the fihras ruituiiig boat
the luller lolo liui optic thaUmi sre mdcpeudent of the anterior

ig (rom I

Ulterior i

384 DUVELOPMfiNl OF OKUAJIS IN MAMMALU. [COiF,

Primitively the Mammaliau cerebrum, like that rf
the lower Vertebrata, is quite smootk In some of tie
Hammalii), Mojiotremata, Inseotivora, etc., this condibon
is retained nearly throughout life, while in the majwrityof
Mammalia a more or less complicated system of fissoret

Lateral \'iew of thb Brais op a Calf Embbto or 5 i
(After Mihalkovics.)
The outer nail of the hemiephiirD U removed, so as to give

view of the interior of the loft lateral ventriole.

Aj. cut wall of heuiisphere ; tl. corpus striatuin ; am. hi{ii»-
campus major {comu ammonisj ; d. choroid plexus of
ventricle ; _/7n. foramen of Monro; op. optio tmct; in. ili
fimdibiilum ; mb. mid-brain ; cb. cerebellum ; IV.V. icnti
fourth ventricle ; ps. pons Varolii, close to whudi is tha ti
nerve with Goftserian ganglion.

19 developed on the surtace. The most importAut, si
first formed, of these is the Sylvian fissure. It ariaet <
the timu when the hemispheres, owing to their g.
in &unt of and behind the corpora striata have aesawti
somewhat the form of a bean. At the root of tbd
hemispheres — the hilus of the bean — there ia f<»iiiaii

XIL] HISTOGENESIS.

I riiallow deprossioa which constitutes the first trace of
I the Sylvian fissure. The part of the brain lying in this
sBure is known as the island of ReiL

The fijwures of the cerebrum may be ilivideil into In-n ciIiisseB ;
ft) the prinuliit!, (2) the laxndari/ f\sa\tiea. The primitive fiBaurea
e the first to appear ; thej owe their origin to a folding of the
intire wall of the cerebral vesicles. M&d; of them are tnuinient
ructuree and earlj disappear. The most important of those
riiich peniiBt are the hippocampal, the parie to-occipital, the
> (ill Man and Apes) sulci and the Sylvian fisauros.
iinila/y fissures appear later, aiid are dae to foldii which
knplicate the cortex of the hemispheres ouly.

The ol&ctory lobes. The olfactory lobes, or rhtnen-
iphaJa, are secondary outgrowths of the cerebral henii-
|)tieres, and contain prolongations of the lateral ven-
les, which may however be closed in the adult stat* ;
tey arise at a fairly early stage of development from
! under and aatt'rior part of the hemispheres (Fig.
l!7).

Histo^netic cliangeB. The walls of the brain are
at first very thin and, like those of the spinal cord, are
formed of a number of ranges of spindle -shaped cells,
lu the floor of the hind- and mid-brain a superficial
layer of delicate ner\'e-fibres is formed at an early
period. This layer appears at first on the floor and
sides of the hind-brain, and almost immediately after-
wards on the floor and the sides of the mid-brain.
The cells internal to the nerve-fibres become differen-
I tilted into an innermost epithelial layer lining the
i^ivities of the venlricles, and an outer layer of grey
matter.

The similarity of the primitive itrrangenipnt and
P. * li. 2.'.

3S13 DEVELdPMEST OF ORGANS IK MAMMALU. [LUiT.
Fin. 137.

Skltion thbouoh the Brain akd Olfactobt Oboas c

EUBBTO OF SaHAAUlS.

lA. cerebral henusphereB ; ol.v. olfactoij vesicle ; i^f. ol£»etan
pit; Sdi. Schneiderian folds; 1. olfactory nBr\-e(tliatefer
line has been accideatall; carried through the oerve m i
appear to indicate the braiu) ; pn. anterior prolongatii
piuea! gland.

histological characters of the parts of the brain behind
the cerebral hcmispherea to those of the spinal conl i)
very conclusively shewn by the examination of any gwi
aeries of sections. In both brain and spinal cord tin
white matter forms a cap on the ventral and latoil
parts some considerable time before it extends to tl»
dorsal surface. In the medulla oblongata the
matter does not eventually extend to the roof owing W
the peculiar degeneration which that part undergoes.
In the case of the fore-brain the walls of the heo
spheres become first divided (KdUiker) into n supei£aJ
thinner layer of rounded elements, and a deeper anJ
thicker epithelial layer, and between these the fibro

"i THE EYE. 387

the crura cerebri soon interpose themselves. At a
slightly later period a thin superficial layer of white
matter, homologous witli that of the remainder of the
bruin, becomes established.

The inoer layer, together with the fibres from the
crura cerebri, gives rise to the major part of the whit«
matter of the hemispheres and to the epithelium lining
the lateral ventricles.

The outer layer of rounded cells becomes divided
into (I) a superficial part with comparatively few cells,
which, together with its coating of white matter, forms
the outer part of the grey matter, anil (2) a deeper
layer with numerous cells, which forms the main mass
uf the grey matter of the cortex.

The eyes. The development of the Mammalian eye
is essentially similar to that of the chick (ch. vi.) There
are however two features in its development which de-
serve mention. These are (1) the immense ftetal develop-
ment of tie blood-vessels of the vitreous humour and
the presence in the embryo of a vascular membrane sur-
rounding the lens, known as the mevihrana capsvlo-
puj/illaris, (2) the absence of any structure comparablf •

to the pecten, and the presence of the arteria centralis •

retinae. i

la the invagination of the lens (rabbit) a thin I

layer of mesoblast is carried before it, and is thus D

transported into the cavity of the vitreous humour. "

Id the folding in of the optic vesicle which accom- |

paiiies the formation of the Ions the optic nerve is
included, and on the development of the cavity of the
vitreous humour an artery, running in the fold of i

the optic nerve, passes through the choroid slit into the i

388 DEVELOPMEKT OF ORGANS IN MAilMALIA. [CHAP,

cavity of the vitreous humour (Fig. 12SiKr). The side3
of the optic nerve subsequently bend over, and com-
pletely envelope this artery, which then gives off

e. ejiithelium of cornea : I. lena ;
the side to furm the cornea
tmlia retiuEB ; o/.n. optic ne

\ mesoblast growing in tcota
t. retina j a.c.r. iirt«m OMi-

The figure Nbuws (1) the abaence at this atngo of meaoblaat
between tlie leiis anil the epibloat ; the interval between th*
two hna however hcen miule too great ; (3) tbe arterio centnilia
rotiue firming the vascular cajteule of tbe Jeiis and continuous
with vaaonlar Btmoturwi round tbe edges of the optii! onp.

;lt.] MEMBIIA.X.V CAPSL'LO-PUPILLARIS. .■)«!>

mches to the retina, tuid becomes kiKJwn as Ihi;
arteria centralU retin«E. It is bomologmis witli the
arterial limb of the vaacular loop projecting intii the
vitreous humour in Biuls.

Before becoming enveloped in the optic nerve this
artery is continued through the vitreous humour (Fig.
128), and when it comes in close proximity to the lens
it divides into a number of radiating branches, which
paas round the edge of the lens, and form a vascular
[ sheath which is prolonged so as to cover the anterior
I wall of the lene. In front of the lens they anastomose
vrith vessels, coming from the iris, many of which arc
venoua, and the whole of the blood from the arteria
centralis is carried away by these veins. The vascular
sheath surrounding the lens is the membrana capsulo-
puyiitai-is. The posterior part of it is either formed
ftiroply by branches of the arteria centralis, or out
of the mesohlast cells involuted with the lens. The
I anterior part of the vascular sheath is however enclosed
i in A very delicate membrane, the membrana pupillaris,
continuous at the sides ivith the membrane of Descemetn
The membrana capsulo- pupillaris is simply a pro-
visional embryonic structure, subserving the nutrition
of the lens.

In many forms, in addition to the vessels of the
vascular capsule round the lens, there arise from tho
uteria centralis retinae, just after its exit from the optic
nerve, provisional vascular branches which extend them-
aelves in the posterior part of the vitreous humour.
Near the ciliary end of the vitreous humour they anuK-
tonioee with the vessels of the membrana utp«ulo-pu-
{uUaria.

390 DEVELOPMENT OF ORQANS IS UAH3LA.LIA. [CUAP,

The choroid slit closes very early, and is not per-
forated by any structure homologous with the pecten.
The only part of the slit which can be said to remain
open is that in which the optic nerve is involved ; in the
centre of the latter is situated the arteria centralis
retiuaj as explained above. From this artery there
grow out the vessels to supply the retina, which however
are distinct from the provisional vessels of the vitreous
humour just described, the blood being returned from
them by veins accompanying the arteries. On the
atrophy of the provisional vessels the whole of the blood
of the arteria centralis passes into the retina.

Of the cornea, aqueous humour, eyelids and lacrymal
duct no mention need here be made, the account given in
Part I. being apphcable equally to mammahan embryos.

The auditory orgaiL. In Mammals, as we have
seen to be the case in the chick (chap, vi.), the auditoty
vesicle is at first nearly spherical, and is imbedded in
the mesoblast at the side of the hind-brain. It sood
"becomes triangular in section, with the apex of the tri-
angle pointing inwards and downwards. This apex
gradually elongates to form the rudiment of the cochlear
canal and sacculus hemisphericus (Fig. 129, CC). At
the same time the recessus labyrinthi {R.L) becomes
distinctly marked, and the outer wall of the main body
of the vesicle grows out into two protuberances, which
form the rudiments of the vertical semicircular canals
(7.5). In the lower forms (Fig. 132) the cochlear
process hardly reaches a higher stage of development tlian
that found at this stage in J^lammalia.

The parts of the auditory labyrinth thus eatablislied
Booa increase in distinctness (Fig. 13U); the cochlear

Trasbvehsb Section of thk Hei.d of a F(btal Sbbbp
(16 KM. TN length) in thb reqioh of thk Hind-Brain.
{After asttchar.)

HB. the hiod-brain. Tho aection is somewbat oblique, hence
while on the right aide the connections of the receaauH feetihnli
R.L., and of the commenciag vertical Bemicircular cAnal V.B.,

and of the ductus coohlearis CO., with the carity of the primnry
otic vesicle are seen : on the left aide, only the extreme end of tho
ductiuicoobleariaCC, aud of the Bemiciruular canal r.fi. are shown.

Lying close to the inner side of the otic venicle is seen the
eocbleu gaogliou OC ; on the left side the auditory nerve O' uiii)
its oonnection N with the hlnd-braiD are also shewn.

Below the otic vesicle on either side lies the jugular vein.

:)92 DEVELOPMENT OF OBGASS IN lUSOIALU. [tUUP.

canal {CC) becomes longer and curved ; its inner mi
ctineave surface being lined by a thick layer of columnnr
epiblast The recessus lubjiinthi also increases in
length, and just below the point where the bulgings tn
form the vertical Bemicircular canals are situated, thw
is formed a fresh protuberance for the horizontal *n>i-

fjRCTioH or THE Uead ov A FsT&L Sheef so km. V

LENGTH. (After Btittcher.)
R. V. rccesBua labjrinthi ; KB. vertical aemiciraular cand ; Bi-

bomontal Hemicirciilar canal i CC cochlear canal; O.VlcL-
Ibbt goDglioa.

[.xn]

THE MEMBRANOUa LABmlNTfl.

:W3

circiilor canal. At the same time the central parts of
thu walls of thu flat bulgiiigs of the vertical canals grow
together, obliterating this part of the lumen, but leaving
A canal round the periphery ; and, on the absorption of
tJieir central parts, each of the original simple bulgings
of the wall of the vesicle becomes converted into a true
semicircular canal, opening at its two extremities into
the auditory vesicle. The vertical canals are first es-
tablished and then the horizontal canal.

Shortly after the formation of the rudiment of the
horizontal semicircular canal a slight protuberance be-
comes apparent on the inner commencement of the
Mhleax canal. A constriction arises on each side of
m&e protuberance, converting it into a prominent hemi-
K;^)herical projection, the sacculus hetnispkericus (Fig.
J 131 SR).

The constrictions are so deep that the sacculus is
nly connected with the cochlear canal on the one hand,
nd with the general cavity of the auditory vesicle on
lie other, by, in each case, a narrow short canal. The
r of these canals (Fig. 131 b) is known as the
inalis reuniens.
At this stage we may call the remaining cavity of
Jie ori^nal otic vesicle, into which all the above parts
open, Uie utriculas.

Soon after the formation of the sacculus hemispheri-
cus, the cochlear canal and the semicircular canals
become invested with cartilage The rec'cssus labynnthi
remains however still enclosed in undifferentiated mesi)-
blast.

Between the cartilage and the parta which it sur-
rounds there remains a certain amount of indifferent

3!)i I>EVKLOPMEKT OF ORGAJfS IS MAMMALU. [CBIP,
Fla, 131.

Sbctios THEtuDQR TBS Intchn'al Eab of an Embbtoik

Sbeep 28 uu. I.N LENCTH. (After Bottcber.)
D.M. dura mater; ILF. receBsiis labjrinthi ; H.V.B. pj^bait

vertical aomimiiiilM caiial ; (.'. utriculuB ; Il.B. horimotal

111.] THE ICEJIBRANOUS LABYRINTH. 3f)o

eenuoirciiLir canal ; b. canalis rBiinienn ; a. coruitrictJon b;
niMns of which the saeoulua hamisiilibricua S.ll. ia fortned ;
/. narrowed nponing between sacculua hemiaphericua aiiil
utriculusi C.C. cochlea; CO', lumen of cochlea; K.K.
carUIaginouB capsule of cochlea; K.B. basilar plate; Ck.
Dotochord.

connective tissiie, which is more abundant around the
cochlear canal than around the semicircular canals.

As Roon as they have acquired a distinct counective-
tissiie coat, the semicircular canals begin to he dilated
«t one of their termiuatious to form the ampullx-. At
about the same time a constriction appears opposite tht'
mouth of the recessus labyriuthi, which causes its open-
ing to be divided into two branches — one towards the
iitriculue and the other towards the sacculus hemispheri-
cua ; and the relations of the parts become so altered
ihat communication between the sacculus and utriculus
caa only take place through the mouth of the recesmis
labyriuthi (Fig. 132).

When the cochlear canal has come to consist of two
and a half coils, the thickened epithelium which lineR
the lower surface of the canal forme a double ridgo
from which the organ of Cortt is subsequently de-
veloped. Above the ridge there appears a delicate
cuticular membrane, the membrajie of Corti or mem-
brana tectorta.

The epithelial walls of the utricle, the saccule, the
recessus labyriuthi, the semicircular canals, and the
cochlear canal constitute together the highly complicated
product of the original auditory vesicle. The whole
structure forms a closed cavity, the various parts of
which are in free communication. In the adult the

390 DEVELOPMENT OF ORGANS IN MAMMALIA- [CHAP.

fluid present in thin cavity is known as the endo-
lijmpk.

Ill the mesobluat lying between these parts and the
cartilage, which at this period envelopes them, lymphatic
ajKtcos become ea tab He hod, which are partially de-
veloped in the Sauropaida, but become in Mammals
very important structures.

They consist in Mammals partly of a space sur-
rounding the utricle and saccule and called the vestibulp,
into which open spaces surrounding the semicircular
canals, and partly of two very definite channels, which
largely embrace between them the cochlear canal The
latter channels form the scala ventibuH on the upper side
of the cochlear canal and the scala ti/mpani on the lower.
The scala vestibuli is in free communication with tho
lymphatic cavity surrounding the utricle and saccule,
and opens at the apex of tlie cochlea into the scala tym-
pani. The latter ends blindly at the fenestra rotunda.

The fluid contained in the two scalie, and in the
remaining lymphatic cavities of the auditory labyrinth,
is known as perilymph.

The cavities just spoken of are formed by an absorp-
tion of parts of the embryonic mucous tissue between
the perichondrium and the walls of the membranous
labyrinth.

The scala vestibuli is fijrmed before the scala tympani,
and both scalie begin t^i be developed at the basal end
of the cochlea : the cavity of each b continually being
carried forwards towards the apex of the cochlear canal
by a progressive absorption of the mesoblast. At first
both seals are somewhat narrow, but they soon increase
ID ^ixe and distinctness.

Xll.] THE COCHLEA. 397

The (XKhlear canal, which is often known as the
scala media of the cochlea, becomes compressed on the
lormatioQ of the scalte so as to be triangtilar in seclion,
with the base of the tritingle outwards. This base is
irity separated from the surrounding cartilage by a
narrow strip of firm mosoblast, which becumL'S ihe stritt
vascularis, etc. At the angle opposite the base the coch-
lear canal is joined to the cartilage by a narrow istlimus
of firm taaterial, which contains nerves and vessels. This
isthmus subac(]iiently forms the lamina spiralis, separ-
ating the scala vestibiili from the scala tympani.

The scala vestibuli lies on the upper border of the
cochlear canal, and is separated from it by a veiy thin
layer of mesoblaat, bordered on the cochlear aspect by
6at epiblast cells. This membrane is called the Tnem-
brane of Reisaner. The scala tympani b separated from
tbe cochlear canal by a thicker sheet of mesoblast, called
the haaUar membrane, which supports the organ of
Corti and the epithelium adjoining it. The upper ex-
tremity of the cochlear canal ends in a blind extremity
called the cupola, to which the two Bcalse do not for
Bome time extend. This condition is permanent ui
Birds, where the cupola is represented by a structure
k&owD aa the lagena (Fig. 132, II. />). Subsequently
the two scalse join at the extremity of the cochlear
canal ; the point of the cupula still however remains in
contact with the boue, which has now replaced the
.cartilage, but at a still later period the scala vestibuli,
growing further round, sejiarates the cujxila from the
adjoining osseous tissue.

Accessory auditory strnctnres. The development
of the Eustachian tube, tympanic cavity, tympani

I. Fish.

AfflmrtiftL

U. utriculua ; i^. stLOCulus ; VS. utriculas aiiil boocuIub
UBoalis rcimieuH ; R. recessuB labvrinthi ;
uicnt of cochlea; C cochlear canul ; L. lagena ; K. oopokt
nt apes of cochloar catial ; Y. cowal tsac of the vestibulimi of
the cochlear caiml.

membrajie and external auditory meatus resembles that
in Birds (p. 166). As in Birds two membranous fenestrffi,
the fenestra ovalia and rotunda, in the bony inner wall of
the tympanic cavity are formed. The fenestra ovalis
opens into the vestibule, and is in immediate contiguity
with the walla of the utricle, while the fenestra rotunda
adjoins the scala tympani. In place of the columella of
Birds, three ossicles, the malleus, incus and stapes reach
across the tympanic cavity from the tympanic membraue

f^]

THE NASAL ORGAN.

390

to the feDestr& ovalis. These ossicles, which arise
mainly from the mandibular and hyoid arches (vide
p, 403), are at first imbedded in the conuective tissue in
the neighbourhood of the tympanic cavity, but on the
fall development of this cavity, become apparently
placed within it, though really enveloped in the mucous
membrane lining it.

Nasal orgatL In Mammalia the geucral fomiatioii
of the aulc'rior and posterior nares ia the same a» in
Birds; but an outgrowth from the inner side of the
canal between the two openings arises at an early perioti ;
and becoming separate from the posterior nares and
provided with a special opening into the mouth, forms
the organ of Jacobson. The general relations of this
organ when fully formed are shewn in Fig. 133.

Nasal Cavity and Jacobson'b Oroah.
(From Oegenbour.)

cavity ; J. Jacobson's organ : d, edga
ot upper jaw.

40O DEVELOPMENT Of ORGANS TN MAMMALIA. [cHAP,

The development of the cranial and Bpinil
nerves in Matiimiils is as for as is known esseutklU
the same as in the chick, for an account of which »c
p. 123 et seq.

Sympathetic nervous Byatem. The development
of the sympathetic system of both Aves and Mamnialia
lias not been thoroughly worked out. There is how-
ever but little doubt that in Mammalia the main put-
tion arises in continuity with the posterior spinal
ganglia.

The later history of the Bympathetic system is ind-
iiiately bound up with that of the so-called supra-reid
bodies, the medullary part of which is. as we shall see
below, derived from the peripheral part of the syiiipa-
thetic system.

THE ORGAKS DERIVED FROM MESOIiLAST.

The vertebral column. The early development of
the porichordal cartili^nous tube and rudimentary
neural arches is almost the same in Slammals as w
Birds. The differentiation into vertebral and inW-
vertebral regions is the same in both groups; but inst**'
of becoming divided as in Biriis into two segmenis
attached to two adjoining vertebra, the inteTVertebtsl
regions become in Mammals wholly converted into the
intervertebral iigaraenta (Fig. 135 It). There are thrw
centres of ossification for each vertebra, two in the arch
and one in the centrum.

The fate of the notochord is in important respeds
different from that in Birds. It is first constricted in
the centres of the veiiehrae (Fig. 134) and disappear!
there shortly after the beginning of ossification ; while iii

XII.]

THE SKULL.

401

tbe intervertebral regions it remains relatively uncon-
itricted (Figs. 134 and 135 c) and after undergoing
certain histological changes n^niains through life as part
of the nucleus pulposus in the axis of the intervertebral
ligatnenta There is also a alight swelling of the iioto-
chord near the two extremitiea of each vertebra (Fig.
13.3 c' and c").

In the persistent vertebral constriction uf the nntuchoitl
Miuuntals retain a more primitive anil piscine mode of formation
of the vertehraJ column tliiui the majority either of the lieptlliu
or Amphibia.

Loaarttmi>'AL Section THaouoa thb Ybbjeboaj. Colcmx
OF A» EiQHT Weeks' fIcuA» EuiiBiro ix thk Tho-
RACto Beoion. (From KOIUker.)

*. uartila^noua vertebral bod; ; li. iuterverteln-ol ligament ;
c/i. nolothorii.

Tll0 skolL Excepting in the absence of the inLur-
orbital plate, the early development of tbe Mamma'
Uan a-anium resembles in all eaaential points that ot'
Aves, to our account of which on p. 233 et aeq. we rdfer
the reader.

F. & U. "ift

402 DEVELOPMENT OF ORGANS IN UAMUALLi. [cHlf.
Fio. 135.

LoNorrCDINAL SecTIOK THEOUOH the iHTKRTKRTEBiUl LlOi-
UBNT AHD ADJACENT pARTB OP TWO VEBTKBtLK FBOM THI

Thokacic Bioion of ax akvahcgd Eubrio or a Sbof.
(From KeUiker.)
[a. liguiiieDtiini longitudinale anterius ; Ip. ligamentum lonf;. p*-
teriiw ; li. ligaineutum int«rvertebrale ; it. If. epiplifiiii "^
vertebra ; v. and ur'. anterior and posterior vertebrw ; <^ '«■
tervertebrai dilatation of uotoobord ; c' and e". nrtobaiH-
Intation of uotochord.

The early changes in the developmeDt of the rwenl
archee and clefts have aJready been described, but the
later changes undergone by the skeletal elementa of tht
first two visceral arches arc sofficiently striking to nW*
a special description.

XII.] MANDIBULAR AND HYOID ARCHES. 40^

The skeletal bars of both the hyoid and mandibular
arches develop at first more completely than in any
of the other types above Fishes ; they are articulated to
each other above, while the pteiygo- palatine bar ia
quite distinct.

The main features of the aubsequeut development
are undisputed, with the exception of that of the upper
end of the hyoid, which is still controverted. The
following is Parker's account for the Pig.

The mandibular and hyoid arches are at first very
similar, their dorsal ends being somewhat incurved, and
articulating together.

■ la a somewhat later stage (Fig. 136) the upper end
of the mandibular bar (wiIi), without becoming segmented

ICmdRVo PlO, AK INCH AND A THIRD LONG ; SlDE VlEW or

Mahdibulak and Hvuid Abcheb. The Main Hyoid

Arch is bees as disphced backwards aftsr Sbombn-
TATioN CROM THE Iscus. (From ParkKr.)

tg. ton^B ; mt. Meckelinn cartilage ; ml. hoAy of malleuH ; mb.
aa or handle of the moUeiix ; C.lg. t«giiieii tympaoi ;
It. Btapes ; iJiy. interhyal ligament ; il.h. stj/lohyal
irtilagti ; h.h. hypoh<ral ; b.h. basihrniicliial ; lk.h. nidiment
rf flrat brsQcbial arvb ; 7a. facial nerve.

404 DEVELOPMENT OF ORGANS IN HAHMAI.JA [CHAP.

from the ventral part, becomes distinctly swollen, and
clearly corresponda to the quadrate region of other typos.
The ventral part of the bar couatitutca Meckel's carti-
la«e (»,*■)■

The liyoid arch has in the meantime become seg-
mented into two parts, an upper part (i), which eventually
becomes one of the small bones of the eai' — the incus —
and a lower port which remains as the anienw comu
(if the kyoid (stji). The two parts continue to be con-
nected by a ligament

The incus is articulated with the quadrate end of
the mandibular arch, and its rounded head comes in
contact with the stapes (Fig. 136, at) which is segmented
from the fenestra ovalis.

According to some authors the stapes is iiuIopcuJetitly fbrmod
from mBDoblaat cells surrouuding a branch of the internal carotid

The main arch of tha hyoid becomes divided into
a hypohyal {h.h) below and a stylohyal (stJi) above, and
also becomes articulated with the basal element of the
arch behind {bh).

In the course of further development the Meckelian
part of the mandibular arch becomes euvGloi>ed in a
superficial ossification forming the dentary. Its upper
end, adjoining the quadrate region, becomes calcified
and then absorbed, and its lower, with the exception of
the estreme point, is ossified and subsequently incorpo-
rated in the dentary.

The quadrate region remains relatively stationary in
growth as compared with the adjacent parts of the skull,
and finally ossifies to form the malleus. The prooesKOs

THE AUDITORY OSSICLES.

403

icilis of the malleus is tLe primitive continuation into
sckel's rartilage.

'he malleus and incus are at first embedHod in tic

?ctive tissue adjoining the tympanic cavity, which

1 the Eustachian tube is the persistent remains of

i hyomandibiilar cleft ; and eittemally to them a bono

II as the tympanic bone becomes developed so that

(Come placed between the tympanic bone and thf

3 capsule. In late fcetal life thoy become trans-

1 completely within the tympanic cavity, though

I by R reflection of the tynijianic mucous mem-

; doTsal end of the part of the hyoid Mpparatod
I the incus becomes ossified as the tyinpaiio-liyal,
Rod is anchylosed with the adjacent partJt of the periotic
i-apsule. The middle part of the bar juiit outMdi^ the
skull forms the stylo-hyal (styloid procesa in man) whicii
is attached by ligament to the anterior coma of the
hyoid (cerato-hyal). The tympanic meinbrsnc and ex-
t«rtjal auditory mestos develop as in the ehick Cp. iW\.

Hm ribs uiid ttarmmm anxv to derdo]> kt Mitnnul* w b
Birds Fp- S^j-

TIm pect«nl Cii<k, m m BMt (p. 04), wwm m a mm-

-«Hm

imm

406 DEVKLOPMENT OF ORGANS IN MAMMALIA. [iVtl.

The skeleton of tlie limbs develops so fai- aa is knowo *• ii
Birds, from a coirtinuoua mesobiastic blaatema, within whicbtha
oorreapoading ciLrtilagiuous elements of the limbs beooUB iltT-
forentiated.

Tlie body cavity. The development of tho M;
cavity and its subsequent division into pericanliil
pleural and peritoneal cavities is precisely the ntute in
Mammalia as in Aves (p. 264 et seq.}. But in Mam-
malia a further change takes place, iu that by the fur-
mation of a vertical partition across the body cavilj,
known as the diaphragm, the pleural cavities, conluD-
ing the lungs, become isolated from the remaiiMia «f
the body or peritoneal cavity. As shewn by iheii
development the so-called pleune or pleural sacs an
simply the peritoneal linings of the anterior divisi>iM
of the body cavity, shut off from the remainder of tlie
body cavity by the diaphr^m.

The vascvlar system.

The heart. The two tubes out of which the heart
is formed appear at the sides of the cephalic plata
opposite the region of the mid- and hind-brain (Fig.
III"). They arise at a time when the lateral fold*
which form the ventral wall of the throat are only jusi
becoming visible. Each half of the heart origiiiat*'^ in
the same way as iu the chick ; and the layer of llw
splanchnic mosoblast, which forms the muscular wall for
eaeh part (ahh), has at tirst the form of a half tubo op>->i
below to the hypoblast.

On the formation of the lateral folds of the splanchnic
THiIls, the two halves of the heart become carriod ini

' Hl] arterlal system.

I and downwards, and eventually meet oa the ventral
B ode of the throat. For a short time they here reiuaia

dijstiBct, but soon coalesce into a siugle tube.

In BirJa, it will be remembereil, the heart at first has tho

form of two tubes, which however are in coutaot in frout. It

»ariaee ut a time when the fommtiou of the thront ia very much
more advanced than iu Moumalia ; when in fact the ventral
mUl of the throat is eatablisbed as far back aa the ftvnt end of
the heart.

In the lower tjpea the heart does not appear till the ventral
wall of the throat is completely eatAbliuhed, and it haii frum the
fint the form of a single tube.

til is therefore probable that the formation of the heart as two
Miities ia a eeeonilary mode of development, which haa been
brought about hjr voriationa in the period of the cloaiiig in of the
TaU of the throat

The lat«r development of the heart ia in the main nmilar to
that of the chick (p. 256 et aeq.).

The arterial system. The early stages of the
arterial system of Maiiiinalia are similar to those in
BirdsL Five arterial arches are formed, the three poste-
rior of which wholly or in part persist in the adult.

The bulbus arteriosus is divided into two (fig. 137
\ B), but the left fourth arch (e), instead of, as in Birds,
I the right, is that continuous with the dorsal aorta, and
I the right fourth arch (i) is only continued into the right
I TCrtebral and right subclavian arteries.

The fifth pair of arches which is continuous with
Ine of the divisions of the bulbus arteriosus gives origin
D the two pulmonary arteries. Both these however are
srived from the arch on one side. viz. the left (fig. 137
B); whereas in Birds, one pulmonary artery comes from
the left and the other from the right fifth arch (fig.
137 A).

DEVELOPMBST OF ORGANS IS MAMMALIA. [CHAP.

The ductus Botalti of the fifth arch (known in Man
ns the ductus art^riusuD) of the side on which the
]>ulmunary arteries are fonncd, may remain (e.g. in Uan)
as a solid cord connecting the common stt^'m of thu
pulmonary aorta with the systemic aorta.

The diagram, Fig. 137, copied from Rathke, shews
at a glance the character of the metamorphosis the
arterial arches undergo in Birds and Mammals,

DCAORAUa ILLDSTRATINO TSE MeTAHORPBOHLS OF TRB AR-
TERIAL Archcs in a Bird A. and a Maumal B.
(Frrnn Mivart nftor Rathke.)

A. a. internal carotid ; b, external carotid ; e. common carotid ;

d, Bjfltemic anria ; e. fourth arch of ri^ht side (root of dontal
aorta) ; / right Bubclavian ; g, dorNat aorta ; A. left sabcln-
vian (fourth arch of left aide) ; i. puImoDarv arterv ; it. and
I. right and left ductus Botalli of pulmooar/ arteriea.

B. a. intenia! carotid ; b. external carotid ; c. cnmmon carotid ;

d. Bystemic aorta ; e. fniirtli arch of left side (rciot of doreal
aorta) ; /. doreat iiorta ; y. left vertehral artery ; h. left aub- ,
clavian artery ; t. right aubclavian (fourth arch of ri^t
Bide) ; t. right vertihral ; I. coiitiTiuation of right Babcla-
vian ; m. pulmansry artery ; n. ductus Botalli of pulmonary
artery.

XH.] VENOUS SYSTEM. 409

In some Mammals both aubclaviana spring from
a trunk common to them and the carotids (arteria
iuionyma) ; or as in Man and some other Mammals,
the tftft one arises from the systemic aorta just beyond
the carotids. Various further modificatious in the origin
of the subclavians are found in Maniuialia, but they
need not be specified in detail. The vertebral arteriea
arise" in close connection with the subclavians, whereas
ill Birds thej arise from the common carotids.

The venous system. In Matnmalti the same venous
trunks are developed in the embryo as in Birds (Fig.
138 A), The anterior cardinals or external jugulars
form the primitive veins of the anterior part of the
body, and the internal jugulars and anterior vertebrals
are subsequently formed. The subclavians (Fig. 138
A, s), developed on the formation of the anterior limbs,
also pour their blood into these primitive trunka In
the lower Mammalia (Monotrcmata, Marsupialia, Insce-
tivora, some Bodentia, etc.) the two ductus Cuvieri
remain as the two superior venje cavE, but more usually
an anastomosis arises between the right and left in-
nominate veins, and eventually the whole of the blood
of the left superior cava is carried to the right side, and
there is left only a single sui>erior cava {Fig. 138 B and
C). A small rudiment of the left superior cava remains
however a-i the sinus cormiarins and receives the coronaty
vein from the heart (Figs. 138 C, cor and 139 cs).

The posterior cardinal veins form at first the only
veins receiving the blood fi-om the posterior part of the
trunk and kidneys; and on the development of the hind
limbs receive the blood from them also.

An unpaired vena cava inferior becomes eventually

410 DEVELOPIIEST OF OKGAKS IN MAMMALIA, [i.!!*!'

-di b

Diagram or ths Develofvknt or ths Paired Vsssn

Ststem ok Mamhalb (Han). (From Oegenbaur.)
/ jugular veio ; ci. rena oava superior; j. eubclavian veim; c
posterior utnlina] vein; p. vertebral vein ; oi. uygna tein ;

ear. coroiiurj' vein.

A. Sta^ in which the cardinal veiau havs alreadf <lis3p-
IHMreJ. Their positioa iis iadicated hy dotted hnea.

B. Lat^r Bti^e when the blood from the leR. jugular vein is
carried into the right to form the single vena cava suimrior ; t
remnant of the left superior cava being however atill left.

C. Stage after the left vertebral vein has diaappearei) ; tU
right vertebral remaining as the iizygOB vein. The coronor; fein
remains aa the laat remnant of the left superior vena cava.

developed, and gradually carries off a larger and larger
portion of the blood originally returned by the posterior
cardinals. It unites with the common stem of Uie
allantoic and vitelline veins in front of the Uver.

I later period a pair of trunks is ostablisb^l
biinging the blood from the posterior part of the cardiiul
I and the crural veins directly into the vooft

VERTEBRAL VEINS.

I Inferior {Fig, 139, il). These vessels, whose development
I lias not been adequately investigated, form the common

DuoRAM OF THB Cbim VEsoua TRfisKs of M*s.
(From Gegenbaur.)
L coronary sinuB ; (. subckvian vein ; JL intenial jiiguli
Je. ext«nial jugulur ; ai. BxygaB vein ; ha. htiiiiiaxjgM lei
c dotted linQ showing previouB position of cardinal veil
ei. vena cava iortnior ; r. renal veius ; H. iliac ; /<y. by pogcw-
tric veiua ; A. hepatio veiiis.

: vessels

iliac veins, wliile the posterior ends of the cardinal veins
which join them become the hypogastric veins (Fig.
I 130 h).

B Posterior vertebral veins, similar to those of Birds,
Kire established in connection with the intercostal and

H2 DEVELOPMENT OF ORGANS IN' MAMMALIA. [CHAP.

lumbar veins, and unite anteriorly with the &ont part
uf the posterior cardinal veins (Fig, 138 A),

Upon the formation of the posterior vertebral veins,
and upon the inferior vena cava becoming more im-
portant, the middle part of the posterior cardinals be-
comes completely aborted (Fig, 139 c). the anterior and
posterior parts still persisting, the fonner as the con-
tinuations of the posterior vertebrala into the anterior
vena cava (as), the latter as the hypogastric veins {hy).

Though in a few Mammalia both the posterior vert^-
hrals persist, a transverse connection is usually established
between them, and the one (the right), becoming the
more important, constitutes the azygoa vein (Fig, 139
ttz), the persisting part of the left forming the hemi-
azygos vein (ha).

The remainder of the venous system la formed in the
embryo by the vitelline and allantoic veins, the former
being eventually joined by the mesenteric vein so as to
constitute the portal vein,

' The vitelline vein is the first part of this system
established, and divides near the heart into two veiiui
bringing back the blood from the yolk-sac (nrabilical
Vfsicie). The right vein soon however aborts.

The allantoic (anterior nbdoiuinal) veins are orig;in-
ally paired. They are developed very early, and at first
course along the still widely open somatic walls of the
body, and fall into the single vitelline trunk in front.
The right allantoic vein disappears before long, and the
common trunk formed by the junction of the vitelline
and allantoic veins becomes considerably elongated.
This tnmk is soon enveloped by the liver, and later in
its passage through, gives off teanches to, and abo

I Xll.j St'PHA-REXAL BODIES. 413

receives branches Irom this organ near it^ anterior exit.
The main trunk is however nevur completely aborted, as
in Che embryos of other types, but remains as the ductus
MMosus ^ran^t.

With the development of the placenta the allantoic
vein becomes the main source of the ductus venosus,
and the vitelline or portal vein, as it may perhaps be
now convenientty called, ceases to join it directly, but
tails into one of its branches in the liver.

The vena cava inferior joins the continuation of the

I ductus venosus in front of the liver, and, as it becomes
more important, it receives directly the hepatic veins
which originally brought back blood into the ductus
venosus. The ductus venosus hecoiuea moreover merely
A smalt branch of the vena cava.

At the close of fcetal life the allantoic vein becomes

I obliterated up to it« place of entrance into the liver ;
the ductus venosus becomes a solid cord — the so-called
round ligament — and the whole of the venous blood is
. brought to the liver by the purtui vein.

Owing to the allantoic (anterior abdominal) vein
having merely a fcetal existence an anastomosis between
the iliac veins and the portal system by moans of the
anterior abdominal vein is not established.

The BUpra-renal bodies. These are paired bodus
lying anterior to the kidneys and are fonncd of two
parts, (Ij a cortical and ('2f a medullary portion. They
first appear iu the Rabbit on the 12th or 13lh day of
gestation, and arise as masses of mesoblaet celU lying
between the aorta and the mesentery and to on« side of

I tJie fonner. On the 14th day they are well marked,

I jtiul lying dorsal to them is another mass of cellg which

414 DEVELOPMENT OF 0EGAN9 IN MAMMALIA. [CHAP.

IK found to be continuous with the sympathetic nervous
system.

Oa the 16th day processes from the sympathetic
mass enter the mesoblastic tissue and become trana-
forraed into the medullary portion of the adult supra-
renal; while the mesoblastic tiaaue gives rise to the
fortical layer.

The urinogenital vrgans.

The history of these organs in Mammalia, escepting
so far as concerns the lower parts of the urinogenital
ducts, is th(? same as in the Chick.

The WolfflEiu body and duct first appear, and aro
followed by the MiiUerian duct and the kidney. The
exact method of development of the latter structures
has not been followed so completely as in the Chick ;
and it is not known whether the peculiar structures
found at the anterior end of the commencing MUUerian
duct in Aves occur in Mammalia.

The history of the generative glands is essentially
the same as in the Chick.

Outgrowths from a certain number of Malpighian
bodies in the Wolffian body are developed along the
base of the testis, and enter into connection with the
seminiferous stroma. It is not certain to what parts of
the testicular tubuli they give rise, but they probably
fonn at any rate the vasa recta and rete vasculosum.
Similarly intrusions from the Malpighian bodies make
their way into the ovary of the female, and give rise to
cords of tissue which may persist throughoiit life.

The vasa efferentia {ci/ni vasculu»i) appear to be
derived from the glandular tubes of part of the Wolffian ,

Xlt.] GEKITAI, CORD, 415

body. The Wolffian duct itself becomes in the male thu
vas deferens and the convoluted canal of the epididy-
mis ; the latter structure except the head being entirely
derived from the Wolffian duct

The functionless remains of the embiyonic organs described
for the chick (p. 224) are found also in iiiammiUa.

The Miillerian ducts persist in the female as the
Fallopian tubes and utenia.

The lower parts of the urinogenitaJ ducts are some-
what further modified in the Mammalia than the Chick.

The genitd cord. The lower part of the WolfBttn
ducts becomes enveloped in both sexes in a special cord
of tissue, known as the genital cord (Fig. 1403c),withiu
the lower part of which the Miillerian ducts are also
enclosed. In the male the Miillerian ducts in this cord
atrophy, except at their distal end where they unite to
form the uterus masculinus. The Wolffian ducts, after
becoming tlie vasa deferentia, remain for some time
enclosed in the common cord but afterwards separate
from each other. The seminal vesicles are outgrowths of
the vasa deferentia.

In the female the Wolffian ducts within the genital
cord atrophy, though rudiments of them are for a long
time visible or even permanently persistent. The lower
parts of the Miillerian ducts unite to form the vagina
and body of the uterus while the upper become the
horns of the uterus and the Fallopian tubes. The
junction commences in the middle and extends forwards
and backwards ; the stage with a median junction being
retained jmrmanently in Marsupials.

The orinogenital sinus and external generative
The dorsal part of the cloaca with the aliinen-

+16 DEVELOPMtINT OF ORGANS IN MAMMALIA. [iJHAP.

tiiry tract becomes partially ooostricteil off from the
ventral, which thco forms a urinogeoital siuus (Fig. 140
ug). In the coune of development the urinogcaiital

Fia, 110.

DiAORAU OF ms Uhihoobnital Oroans or a Uamual at

AH Earli Stage. (After Allen ThomBun ; from Quiuq's
jlnutui"^,)
The [larts ore aeea chieSy in profile, but the Uulleriaii and

Wultliaii (iiicta ore seea from the fronL

3, ureter; 4. urinury bladder; 5. uracUuH ; o(. genital ridge
(ovary or tattLs); W. left Wolffian body; «. jtart at apex
from nhiuh uuiii vasculoai are afterwards developed ; w.
Wolffian duct ; ni. UUllerioD duct ; gc. gonitiU cord uoQsist-
iug of Wolffian and MiUlerian ducts bound up in a common
aheatii ; i. rectum ; vg. ufinogeiiital aiuua ; ep. olevatiou
which becomes the clitoris or penis ; l*. ridge from which the
labia miyora oi aciutum are developed.

ill.] EXTERNAL GENERATIVE ORGANS. 417

Rmus becomea, in all Mammalia but the Omithodelphia,
completely separated from the intestinal cloaca, and the
two parta obtmn separate external openings. The
ureters (Fig. 140, 3) open higher up than the other
ducts into the stalk of the allantois which here dilates
to form the bladder. That part of the stalk which con-
necta the bladder with the ventral wall of the body
constitutea the urachua, and losea its lumen before the
close of embryonic life. The part of the stalk of the
allantoia below the openings of the ureters narrows to
form the urethra, which opens together with the Wolffian
and Mullerian ducts into the urogenital cloaca,

In front of the urogenital cloaca there is formed
a genital prominence (Fig. 140 c^) with a groove con-
tinued from the urinogenital opening, and on each side a
genital fold (Is), In the male tho sides of the groove on
the prominence coalesce together, embracing between
them the opening of the urinogenital cloaca, and the
prominence itself gives rise to the penb, along which the
common urinogenital passage is continued. The two
genital folds unite from behind forwards tx) form the
scrotum.

In the female the groove on the genital prominence
gradually disappears, and the prominence remains as the
clitoris, which is therefore the homologue of the penis:
the two genital folds form the labia majora. The urethra
and vagina open independently into the common .uro-
genital sinus.

THE ALIMENTARY CANAL AND ITS APPENDAGES.

It is convenient to introduce into our account of the
organs derived fr^m the hypoblast, a short account of
F. 4 D. "R

41S DEVELOPMENT OF ORGANS IN KAIHLALIA. [cHAP,

certain organs connected with the alimentary canal
such as the mesentery, stomodteum, etc., which ore not
bypoblastic in origin.

The origin of the hypoblast, and the procea of
folding by which the cavity of the mosenteroa is
established have already been described. The inesen-
teron may be considered under three heads.

1. The anterior or respiratory division of tht
viesenteron. The pharynx, thyroid body. Eustachian
tube, tympanic cavity, oesophagus, trachea, bronchi, lungs
and stomach are developed from this portion, and thar
development in the Mammal ao closely resembles that in
the Chick that it is unnecessary for ua to add to tlie
account we have already given in the earlier part of this
work.

This section of the alimentary canal, as in the Chick,
is distinguished in the emhiyo by the fact that its walls
send out a series of paired diverticula which meet th"
skin, and, after perforation has been effected at the
regions of contact, form the visceral clefts.

2. The middle division of the viesenteron, bom
which the liver and pancreas are developed, as in the
Chick, forms the intestinal and cloacal region and is «
first a straight tube. It remains for some time connected
with the yolk sack.

The Cloaca appears as a dilatation of the meaen-
teron which receives, as in Aves, the opening of the
allantois almost as soon as the posterior section of
the alimentary tract is established, The eventual
changes which it undergoes have already been dealt
with in comiection with the urinogenital organs.

The mtestine. The posterior part of this becoms

XU.] THE MESENTEKV. 419

enlarged to form the large intestine, wliilo tlie anterior
portion becoming very much elongated and coiled forms
the Bmall intestine, and moreover gives rise anteriorly
to the liver and pancreas.

From the large iatcstine close to its junction with the amall
intestiae an outgrowth is developed, the proximal port of which
enlarges to form the cacam, while tho distal portiou in Man
tonaa the vermiform appeitdix.

3. The postanal division of the mesenteron atro-
phies at an early period of embryonic life. In the Chick
and lower types it communicatea for a short time with
the bind end of the neural canal,

Sjdanchiiic mesoblast and mesentery. The mesen-

I teron consists at first of a simple hypoblastic tube, which

wwever becomes enveloped by a layer of splanchnic

wbUst This layer, which is not at first continued

jwer the dorsal side of the mesenteron, gradually grows

,, and interposes itself between the hypoblast of the

mteron, and the organs above. At the same time

b becomes differentiated into two layers, viz. an outer

pithelioid layer which gives rise to part of the peritoneal

pithelium, and an inner layer of undifferentiated cells

prbich in time becomes converted into the connective

Bsne and muscular walls of the mesenteron. The

^nnective tissue layers are first formed, while of the

Biuacular layers the circular is the first to make its

Coincidently with the differentiation of these layers
e connective tissue stratum of the peritoneum becomes
tabUshed.

The tTiesentei-t/ is developed as in the Chick (p. 172).
1 the thoracic region it is hardly if at all developed.

420 DEVELOPMENT OF ORGANS IN MAMMALIA, [CHAP.

The primitive simplicity in the arrangement of the
mesentery is usually afterwards replaced by a more com-
plicated disposition, owing to the subsequent elongation
and consequent convolution of the intestine and stomach.

The layer of peritoneal epithelium on the ventral
side of the stomach ia continued over the liver, and
after embracing the liver, becomes attached to the
ventral abdominal wall. Thus in the region of the liver
the body-cavity is divided into two halves by a mem-
brane, the two sides of which are covered by the peri-
toneal epithelium, and which encloses the stomach
dorsally and the liver ventraljy. The part of the mem-
brane between the stomach and liver ia narrow, and
constitutes a kind of mesentery suspending the liver
&om the stomach : it Is known to human anatomista as
the lesser omentum.

The part of the membrane connecting the liver with
the anterior abdominal wall constitutes the falciform or
suspensory ligament of t}ie liver. It arises by a secondary
fusion, and ia not a remnant of a primitive ventral
mesentery (vide p. 2Gi).

The mesentery of the stomach, or mesogastrium,
enlarges in Mammalia to form a peculiar sack known as
tiie greater omentum.

The stomodiBiun. The anterior section of the per-
manent alimentary tract is formed, as in the Chick, by
an invagination of epiblast, constituting a more or leas
considerable pit, with its inner wall in contact with the
blind anterior extremity of the mesenteron.

From the epiblastic lining of this pit are developed
the pituitary body and the salivary as well as the other
buccal glands.

DlAQttAM Shbwinq the DIVISION OF THB PantiTrvE Boccal
Cavity isto thk Respiratoby Section above akd the
TBOB Mouth below. (From Qegeabaur.)

palatine plate of superior maiillarj process; m. permanent
mouth; n. posterior port of nasal passage; a. intomasal
eq>tam.

A palate grows inwards from each of the superior
miliary processes (Fig, 141), which, meeting in the
iddle line, form a horizontal septum dividing the front
lit of the BtomodiBum into a dorsal respiratory section,
mtalning the opening of the posterior nares, and a
(ntral cavity forming the permanent mouth. These
divisions open into a common cavity behind. This
itum on the development within it of an osseous
Lte constitutes the hard palate. A posterior pro-
,tion in which no osseous plate is formed eonsti-
itea the soft palate. An intemasal septum (Fig. 141 e)
lay more or less completely divide the dorsal cavity
ito two canals, continuous respectively with the two
' cavities.

Tlie teeth are special products of the oral mucous
membrane. They are formed from two distinct organs,
viz. an epithelial cap and a connective tissue papilla,

422 DEVELOPMENT OF QRQAKS IN iffAiruATJA [m.

which according to most authors give rise to the enamel
and dentine respectively.

The proctodSBUm. The cloacal section of the ali-
mentary canal is placed in communication with the
exterior by means of a shallow epiblastic invaginatioD
constituting the proctodseum.

APPENDIX.

PBACTICAL INSTRUCTIONS FOB STUDYING THE DE-

VELOPMENT OF THE CHICK.

I. A. Incnl)ators.

Of all incubators, the natural one, i,e. the hen,
is in some respects the best. The number of eggs
which fail to develope is fewer than with an arti-
ficial incubator, and the development of monstrosi-
ties is rarer. A good sitter will continue to sit
for thirty or more days at least^ even though the
eggs are daily being changed. She should never
be allowed to want for water, and should be well
supplied according to her appetite with soft food.
It is best to place the food at some little distance
from the eggs, in order that the hen may leave
the eggs when feeding. She will sit most per-
sistently in a warm, quiet, somewhat darkened
spot. When an egg is placed under her, the date
should be marked on it, in order that the duration
of its incubation may be exactly known. When
the egg is intended to remain for some time, e.g.
for seven days or more, the mark should be bold
and distinct, otherwise it will be rubbed off.

PRACTICAL DIRECTIONa [APP,

On the whole however we have found it man
convenieat to use a good artificial incubator. Vt
have ourselves used with eucceaa two diflisitiit
incubators. One made hy the Cambridge Scientific
Ingtrument Company, and the other by Wiesntgg
of 64, Rue Gay-Lussftc, Paris (Fig. 65 in Mi
cfttftlogue for IBPl). We have had the longwtM-
perience with the former, and have found it woA
exceedingly well : having been able to hatch ducki
without more attention than now and then turniDg
over the eggs.

Both these incubatora couBisl essentially of i
large water-bath fitted with a gas regulator. Tbey
are both perfectly automatic and when once re-
lated require no further attention.

The temperature within the incubator ahoaU
be maintained at from 37° to 40°O. A rise abate
40" is fatal ; but it uiay bo allowed to descend to
35° or in the yoimg stages lower, without doing
any further harm than to delay the development
The products of the combustion of the p*
should be kept as much as possible &om the ffp.
while a supply of Jreih air and of moiMnrt >•
ementiaJ.

Toletttblj Batisractor; rL>BiiIta maj' be olilamed *illi
aa ordinary chemical double-jacketed diTiiig w&tai*lMtl),
thoronghlj ooverad in with a thick eoat of cotton
sad Bannel baize, and be&ted by a very small gti-jil-
If the vesKi be filled mth hoi water, and allowed U
down to 40° or tliercabouls, before the ^gs are inlrod
a vety small gas Snms will be stiffleieot to mainlai:
requisite temperalurc A small pin-hole-ODizle, piiol
with ordinary presauro an exceeding narrow jet of ft
about two inches high, is the most convenient. B; b
ing (be gas q5 or on, so as to reduce or inereaes UithogU

HARDENING EMBRYOS. 425

of the jot fts required, & v«rj stead; meiui temperature
maf bo maintained.

In the absence of gas, a patent oigbt-ligbt placed at a
proper distance below the bath ma; be mode to ausKcr
Tery well, When a body oF water, once raised to the
neoesaar? temperature, is thorouglilf surrounded with
non.ccnduiitLng material, a ver; alight constant amount ol
heat will supply all the loss.

B. On preparing sections of tlu: cmbtyo.
Habdeniko.
0. Picric add.

We find this reagent thu moat satiafactoiy
for hardenitig the chiak tiad in most instances
mammalian eiabryoy.

Kleinenberg's solution of picric acid is the
\ best.

With 100 parta of water, make a cold
Batnrated solution of picric acid ; add to thia
two parts of concentrated Bulphurio acid or
nitric acid : filter and add to tlie filtrate three
times its bulk of water.

In thia solution of picric acid' the embryo
must be placed and left for from 2 — 5 houis.
It should then be washed in alcohol of 30 p.o.
and [ilaced in alcohol 50 p.c. for ons hour.
From thia it must be removed into alcohol
of 70 p.c. in which it should bo left until
all the jiicric acid is extracted ; to facilitate
thia the TO p.c. alcohol should bo frequently
changed : whan free from picric the embr*

^ It \% sometimes advaatageotu to add to this solation of I
d ai maoh pure krcasote as it will dissolve (viit Sleiiv
' Development of Earthwami," QuarUrlg Journal of 31ie.

1
I

PRACTICAL DIRECnOSS. [APP.

should be placed is 90 p.a alcohol and kept
there until required for further use.

N.B. Hardened enibiyos should alwaji b«
kept in 90 p.c. spirit and only placed in riw-
lute before imbedding, or Etaiuing with haeow-
toxylin.

Some hietologtsta prefer to keep hardeiied tinna
ia alcohol 70 p.c.

l>. OoTTosive Bahlimat«.

Place the embryo in a large quantity of »
saturated aqueoua solution of corro^re sobli-
mat« to which a few drops of glacial acetic acid
have been added, and allow it to remain for
half-an-hour'. It ia necessary thoroughly tow-
tract the corrosive sublimate from the cells of the
embryo; to accomplish this, wash it thorougbly
with water for from 10 minutes to 3 hours &►
cording to the size of the object. The washing
may be limited to frequent changes of water or
the embiyo may be placed in a vessel throngb
which a continuous stream of water ia kepi
running. When all the sublimate is remored,
place it in 50 p.c. alcohol luridtdatcd with nitric
Hcid (halfa-dozeu drops of acid to a 4 oe
bottle of spirit) for five miuutea. The preser-
Tation of the embryo ia completed by treating
it with 70 p,c. alcohol for twenty-four hours Mil
then keeping it in 90 p.c. alcohoL We hit™
not found that corrosiTe sublimate gives sach
good results as pioric acid in the case of chickt
and mammalian embryos.

If tliero is only a Bmoll quantit; of acftio acid mixed with tbt
SUtle, a prolonged immeraioa will do the ombiytt no ham.

HAHDENINO EMBRYOS,

Osmie add.

Oamic acid is a difficult reagent to use, but
when properly applied it gives moBt excellent
results.

It should be used as a weak solution ('1 to
■5 p.o.}. The object ehould be left in it until
it has acquired a tight brown tint The stronger
the solution the leas time ia required for the
production of this tint. It should then be
removed and placed in picro- carmine, which
arrests the action of the oeniic and stains tha
embrya The time required for the picro-car-
mine staining must be determined by practice.
From the picro-carmine the object murt be
washed in 70 p,c. spirit; and then placed iu
90, or may be presen-ed directly in glycerine.

If it is desired to use other staining agents
(borax-carmine ia good for some preparations),
the object must be removed from osmic into
water or weak spirit, tbence through 50 into
70 p.c, stained, and passed through TO to
90 p.c. spirit

After using osmic it is well in some cases
{mammalian segmenting ova) to place the
object in Muller'a fluid for 2 or 3 dsys, after
which it may be preserved in glycerine or spirit.

Muller's fluid is made by dissolving 25 gnns.
of bichromate of potash and 10 grms. of sodic
sulphate in lOOO cc. of water.

With chromic add.

The embryo must be immersed in a solution
of the strength of -1 p.c for 24 honrs. From
this it should be removed and placed in a stronger

PRACTICAL DIRECTIONS. [APP.

Bolutiou ('3 p.c.) fur another 2i hours. If it
then appears Bu£ciently hard, it may bo at
once placed in alcohol of 70 p.&, in which it
should remain for one da;, and then be trans-
ferred to alcohol of 90 p-c,
: Absolute alcohol has also been emplojred aa

a hardening reagent, but is by no meana so good
aa the reagents recommended above.

The olijoct of these so-called bardeniDg teageata ia
b} HU the tiesueB with tho greatest possible rapidity
without thereby destroj'ing them. The aabBeqiient
treatment vith alcohol completes the hardening which
is only oommenoed by these reagents.

There is room for the eieroiso of oonsiilcrable skill
in the use of aloobol, and this skill can onlj bo acqnired
bj eiperienco. A few general rules may however be
Uiddown.

(1) Tissues should not, generally, be changed Erom water
or an aiiueoas solution ot the first hardening reagent
into an alooholic solation of too great strength, nor
should the Bocoessive solutions of alcohol used differ
too mueh in Btrengtb. The distortion produoed by
the violence and inequality of the diSosJon cnnenta
is thus diminished. This general rule should be
remembered in transferring tissues from alcohol to
the staining agents and vice versa.

(2) Ibe tisgaes should cot be left too long (more than
one or two hours) in alcoholic solutions containing
less than TO p. a. of alcohol.

(3) They should not be kept in absolute alcohol longer
than is neoessary to dehydrate them (see B. 1, p. 436).
The alcoholic solutions we generally nse contain 80.
eo, 70, 90 p.c. of alcohol.

Stain ujo.

In moet coses it will be found of advantftge
to Btoin Maa emhrya. The best method of doing

STAINIKO EMBRYOS.

this ifl to Etuin the enibiyo as a whole, rather 1
than U> Btam the individual Hectiona an«r they I
have been cut.

We have found hcematoxylin and borax-
carmiae the best reagunts for staining embrjoa I

Witb hamatozylin.

The best Bolution of htematoxjlin, oi
which we are indebted to KJeinenberg, is
in the following way,
l) Iklake a saturated solution of crystallized caU
cium chloride in 70 p-c, alcohol, and add
alum to aatiiration.
[i) Make also a saturuted Bolution of alum in 70
p.c. alcohol, and odd 1 to 2 in tlie proportion
of 1 : 8.
[s) To the mixture of 1 and 2 add a Jisto drops of
a saturated solution of hfematozylin in ab-
solute alcohol.
[4) It is often the case that hnmatoxyliu solutioQ
prepared in this way has not the proper
purple tint ; hut a red tint. This is due to
acidity of the materials used. The proper
colour can be obtained by treating it with
some alkaline solution. We have found it
convenient to use for this purpose a saturated
solution of sodium bi-carhonate in 70 p.a
tspirit, (The exact amount must be deter-
mined by experiment, as it depends upon the
amount of acid present.)
The embryo should be placed for some hoi
in absoluU alcohol, befora attuning with.

4
4

PRACTICAL DIRECTIONS. [iPJ.

matoxyiin, and should be removed directly from
absolute into the hKiua,tAxylm.

The time required for staining rariea wilh
the size of the object and the strength of tt?
staining fluid. Hiematoaylin will not stain if
the embryo is not quite free from acid.

If the enibryo is atained too dark, it ahoolii
be treated nith & solution of 70 p.a tlodboi
acidulated witli nitric acid ('25 p.c. of acid)
until the excess of staining ia removed; mdin
all ca-sea the htematoxylin staining is improved
by treating the embryo with acidulated 70 f-t
alcohol.

AAer Btaining the embryo must be *eU
waslied in 70 and placed in 90 p.c. spirit.

Witli borax-carmine.

Make au aqueous solution of 3 to 3 p-c
carmine and 4 p.c. borax, by heating : add is
equal volume of 70 p.c. alcohol, and let the
mixture staud for thirty-sis hours; after Trhid
carefully filter.

Stain the object thoroughly by leaving it in
this solution for one or even two days; it will
attain a dull maroon colour : transfer it tlien to
acidulated alcohol (see a) until it becomea •
bright red, and afterwards keep it as before in
90 p.c. alcohol.

This staining solution permeates more tlio-
roughly and uniformly a large object than don
hfflmatoxylin ; therefore when a four or five iliy
chick is to be stained, borax-carmine is the best
staining reagent to use. Embryos that htrt
been preserved in corrosive sublimate will be

thorougUly in this Iban i
Golution.

e akolioUc solution is
the beat. Into tliia the embryo may be removed |
directly from 90 p.c. alcohol, left (or 24 hours,
nnd then placeJ aguin in alcohol until required.

With plcro-carmine.

Thia reagent is useful as will be seen later '
for staining mammalian segnienting c
very young blaatodenna ; it is used with the j
greatest success afler hardening in osmic

There are several methoda of making picro- ^
Cftrmine, the following in the simplest, and we
hare found it answer our purpose fairly well.

To a solution made up of 1 grm. of car-
mine 4 cc. of liquor ammonia and 200 cc. of
distilled water add 5 grma. of picric acid ; agitate
the miictiirB for some miuutos, and then decunt, i
leaving the excess of acid. '

The decanted fluid must remain for several <
days, being stirred up from time to time; even-
tually evaporated to dryness in a shrtUow vessel, ]
and to every 2 grms. of the residue add 100 o
of distilled water.

With alum camune.

To make it, boil a strong aqueous solution <rf <l
amtnonia-alum with excess of carmine for 10 to j
20 minutes, filter, and dilute the filtrate uatUJ
> it contains fi:om 1 to 3 p.c of alum. Add ^M

w drops of carbolic acid to prevent the grow
KjOf fungus.

PRACTICAL DIRECTIONS. [AFP,

Well Lardened tissues may be left in tLit
aqueous solution for 24 hours. It is especiAllj
good for Etaiuing nuclei; as a rule tte staining
is sot diffuse, but it is necessarj' after ■tumig
to treat with add alcohol (see a).

lUBEDDIIfO AKD CUTTINO SeCTIOKS.

It is not possible to obtain satisfactory sec-
tions of embryos without employing vmt
method of imbedding, and using a microtomt
Klany imbedding solutions and nietboda of cut
ting sections have been used, but we find lite
following far superior to any other. It combinM
several advantages ; in the first place it rendtn
it comparatively easy to obtain, what is »
essential, a coniplete eonsenUit>e »en<» of BM-
tions of the embryo ; and secondly, all the ««■
tions when mounted ore in the same reUtiw
position ; and tlie various parts of each section
retain their normal position with regud to
each other.
a. Imbedding.

The substance we prefer for imbedding is
paraffin. As will bo snon below it is neoessair
to have at hand paraffins of various melting
points, according to the temperature of tlw
room at tiie time when the sections are cat.

It will be found most convenient to obtain
paraffins of the highest and lowest melting
points and to mix them together as experienw
dictates.

Place the stained embryo in abeolute alco-
hol until completely dehydrated (two boun
sufficient for small embryos) : and when leaij

IMBEDDING. 433

to imbed soak it ia turpcntiDe' until it is com-
pletely saturated ; and transfer it tbence with at
lUtlt turpentine as posaibU to a dish of melted
paraffiu.

Iq rasea of very delicate tissoes, it U better lo oee
Fliiurofuim inHtfad ol turpentiae. The cUorofona
should be carefnllj added by meaoB of a pipette to the
ubaolnte alcohol in nhioh the tissue is pluxd. The
chloToform Einks to the bottom of the bottle or tuba
and the embiyo, trbich at Brst lies at the junction of the
two liquida. gradually aiuks iuto the chloroform. When
this is accompUebed, remove a/I the absolute with a
)ii{«lte and add pieces of solid parafEa to the chloroform.
Uentlf warm this on a water bath till all the chlorofonn
Ib driven oS ; then imbed in the usual way.

Care must be taken that no more beat ■■
used than ia necessary to melt the pamffin ; for
this purpose the puraffin aliouli) be warmed over
a water bath the temperature of which is kept
constant (from 50 to GO''C. but not more than

eo'C).

A paraffin melting at U'C. is of the proper eonaisteac;
tor catting when the temperature of the room ia 1S*C.

With care a porcelain evaporating dish and
a gas tiame may Vie made to answer, but the
ittudent is advised not to imbed without a
wat«r bath.

The embryo may be left in the paraffin two,
three or more hours, after which it is imbedded
by placing it along with the melted [laraffin in
either a bos made by bending up the sides and
folding in the comers of a piece of stiff pa[>er,
or what is better, a box formed by two L-ahaped
a alcohol ia Dot quite abeolate kreasote ahoold be
I of turpentine.

B. is

f pa[>er, t

j-shaped ^^^H
be iue4^^|

PRACTICAL DIKECTIONS.

[tit

pieces of lead, placed on n glass slide in sncli I
manner as to enclose a space. The UUo' is
preferable because the object can be pUccd
iu any position required with great eaae hj
moving it with a hot needle, and the whole cui
be coolud rapidly. It is advisable, at any mt* »t
fit-st, to arrange the embryo so as to cut it inui
leans verse sections.

When cool a block of paraSa is formed, in
the midst of which is the embryo.

Otbci imboddiiig agonts hava been n»cA. The bcM
of Iheaa are, (Ij pure cocoa butter { (2) & aiinn
Bpormooeti and caitor oil or u^ooa butter (1 putt ol
the former to one of the Inttcr). T^tb these imW^
BuMaDces, it is generally necessuytomolgteDtbemuci
either vitb olive oil or turpentine and ribboui of «o-
tionB cauiiot be made (see b).
Outtiug sections.

When the imbedding block is cold pare awij
the edges, then gradually slice it away until tie
end of the embryo is near the Burfaoe, I
place it in a microtome.

The microtome we are moat accDstomeil to i»
a 'sliding microtome' made by Jung of Heidel-
berg; it gives excellent results. Recently bow-
ever Messrs CaJdwoll and Tlirelfall havedetij
an automatic microtome which hns been lufd
with success at the Cambridge MorjiliologiwI
Laboratory and promises to effect a great taving
of time and trouble iu cutting sections (vide p^7l
and I'roceed'mgii of tlie Cavibridge PhU.Soe. I88S).
A convenient small microtome is one made I7
Zeiss of Jena (also by the Cambridge Scientific
Instrument Company), iu which the object it
fixed and by means of a finely divided s

CUTTING SECTIONS.

raised Uirough a hole in a glasa plate, aci'on
which a razor held ia the hand is pushed. W«
will briefly describe the method of manipulatiMi
for the small microtome, it will be found coailj
applicable to Jung's eiiding microtome.

The paraffin block is pared in such a manner
that the edge nearest to the oiwrator and that
opposite to him are paraLel. A dry razor ia
then pushed upon the glass plate over the hole
through which the block of parafiin projects up-
wards, and a section cut which remains upon
the razor. Care must be taken that the edge of
the razor is parallel to the parallel edges of the
paraffiu block. The block having been raised
by the screw, a second section is made in the
same way and on the same part of the razor aa
the first; in consequence of which, the first
lection will be pushed backwards by the second.
Similarly each new section pushes backwards
thoee already made ; and a ribbon of sections
formed which, \f tlie paraffin m of the viijht
tontiitency, will adhere firmly together.

Experience must teach the manipulator how,

' to mix the paraffin in such a manner that it ia

neither too hard nor too soft ; if it is too hard,

< the sections will not adhere together and will

I curl up on the razor, if too soft they will

I Btick to the razor and be found to be creased.

When it ia not possible to keep the temperature

of the room constant it will be found couvenient

a hard pamffin, and when necessary
raise the temperature by means of a lamp.
Ihe paraffin should completely surround
^embryo and fill up all the spaces within it

28— a

sea
.ure

■ to I

PllACTICAL DIRECTIONS. [APF.

Mounting sections.

When tlio sections are cut, place lliem in
rows oiiaalide prepared in the following mauntr.
Make a solution of white ahellac in kreuols
by lioating, and let it be of the canaisteocf of
glycerine, or slightly more fluid. VTitU acamel'i
hair-bruah paiul a very thin and uniform Uy"
of this guDi over tlia slide which must ba daa
and (Irj/, and while the ^iim ia wet place the sec-
tiona in rows upuu it Now place thu slide ou «
water batU which is heated up to the melting
point of the parafiin. The sectionH unk down
into the thin layer of eheUao and kiessote, the
kreaBot« slowly evaporates and the shellac be-
coming hurd fixes the section in the position m
which it was placed on the slides 'Wheo Uia
kreasote has been evaporated, pour turpMitiiie
carefully upon the slide, tlm dissolves the pa-
raffin and clears the sections which may at once

be mounted i

Canada baleam.
ir chloioform solution of Canada balMin

Thia method of cutting ribbons of sectioM
was first introduced by Mr Caldwell, to whooi
we are also indebted for the account girenaWs
for mounting sections (vide Note B, p 4T1).
Tho latter however is a modification and im-
provement of Dr Gieabrecht'a method. (Zoolo-
gUelier Anzeiger No. 92, 1881.)
Preservation of the embryo as a wlwle.

Chii-k embryos of the firat or second day may V"
easily preserved whole aa microscopic objects. For
thia purpose, tho embryo, which has been prcaerrtd

OPESISG THE EGG.

437

II.

ill the ordinary way (B, a) should be stained eliffhtty,
dohydmted, soaked in oil of cIoveB until transparent
and mounted in balsam.

Whole embryos of o. later date cannot be utis-
foctorUy preserved as microscopic objects.

Practical DiREcrioNa fob obtainiscj asd studyino

CUICK EUBRTOa.

Examination of a 36 to 48 hours' embiyo.

The student will find it by far the beat plan to begin
with the study of an embryo of this date. The mantpu-
lation is not difficult ; and the details of structure are
sufficiently simple to allow them to be readily grasped.
Earlier embryos are troublesome to manage until some
experience bus been gained; and the details of later
ones are so many as to i^emler it undesirable to begin
with them.
\. Opening the Egg.

Take the egg warm from the hen or the incu-
bator, and place it (it does not matter in what poHi-
tion, since the blaatoUerm will at this stage always
be found at the uppermost part of the egg) in a
small basin large enough to allow the egg to bu
covered with Hiiid. It is of advantage, but not
necessary, to jiiace at the bottom of the basin a
mould, e.g. a fiat piece of lead with a concavity on
the up]ier surface, in which the egg may rest securely
without rolling. Four into the basin so much of a
■75 per cent, solution of sodium chloride warmed to
aST. as will cover the egg completely. With a sharp
tap break through the shell at the broad end over
the air chamber, and let out as much air as has
already been gathered there. Unless this is As'

PRACTICAI. DIKECTIONS. [APF.

the presence of air iu the air-chamber will cftuse li«
broad end to tilt np. At ttus date there will be
very little air, hut in eggs of longer incubation, in*
uonveaience will be felt unless this plan be a(lopt«d.

Instead of being broken with a blow, the ihell
mtij he filed through at one point, and the opening
enlarged with the foi-cepa; but a little practiiie iriil
enable the student to use the furmer and &am
method without doing damage,

With a bluut pair of forceps, remove the shell
carefully bit by bit, leaving the shell-membnae
behind; begin at tlie hole made at the brood end,
and work over the upper part until about a third or
half of the shell has been removed.

Then with a finer pair of forceps remove thf
shell-membrane 3 it will readily come away in etripK,
torn across the long siis of the egg in a somewhat
spiral fashion. The yolk and embryo will now come
into view.

It is the practice of some simply to break the egg
across and pour the yolk and white together into •
basin, very much as the houeewife doea Wo feel
sure, however, tliat the extra trouble of the method
we have given will be more than repaid by the
results.

During this time, and indeed during the whole
period of the examination of the embryo tn «t(a, the
basin and its contents must be malntcuned, either by
renewal of the salt solution, or by the iMan being
placed on a sand-bath, at about 38*0.

Examination of t/te blastoderm in situ.

This may he done with the naked eye, or with «
simple lens of low power. Observe ; —

REMOVAL OF THE EMBRYO.

«n j

the pellueitt |

Jimn mav Im

1. Lying across the long axis of tlie egg, the p
area, in the middlo of which the eaibryo may bo
obscurely seen as a white etreak.

2. The mottled vascular area, with the blood-vessels
just beginning to be formed,

3. The opaque area spreading over the yolk with the
changes in the yolk around its periphery.

4. (With ftsimple lens), the contractions of the heart;
perhaps the outlines of the head of the embryo
may ba detected,

C. Removal of the embryo.

Plunge one blade of a sharp fine pair of scissors
thi-ough the blaattiderro, just outside the outer margin
of the vascular area, and rapidly curry the incision
completely round until tlie circle is complete, avoid
as much as possible any agitation of the liquid is the
basin.

With a little trouble, the excised blastoderm may
now be floated into a watch-glass, care being taken to
keep it as flat as possible. With a pair of forceps or
with a needle, aided by gentle shaking, remove the
piece of vitelline membrane covering the blastoderm.

If any yolk adheres to the blastoderm, it may with
a little gentle ugitation easily be washed off. Some-
times it is of advantage to suck up the yulk with a
glass syringe, replacing the lluid removed with clean
('75 p.c.) salt solution.

The blastoderm should now be removed from the
waleh-glasa to a microscopic glass slide ; since it in
difficult in the former to prevent the edges of the
blastoderm &'om curliog up.

40 PRACTICAL DIRECTIONS. [iPP,

The transference may easily be effected, if boUi
the watch-glass aud elide are phingcd into a basin of
clean warm salt Bolution. With a little care, lix
blastodet'm can tben be floated from the one to llie
nther, and the gloss slide, hariiig the blaetodenn with
its upper surface nppermoBt apread flat upon it, leiy
gently raiaiid out of tlie liquid.

A thin ring of putty may now be placed round
the blastoderm, a amiill quantity of salt solutioD
gently poured within the ring, and ibe whole cDveml
with a glass slide, which may be pressed down ontil
it ia sufficiently close to the embryo. The preseoce
of any air-bubbles must of course be avoided.

Provided cure be otherwise taken, to keep the
embryo well coi-ered with liquid, the putty ring uj
the coverslip may be dispensed with. They are often
inconvenient, aa when the embryo has to be tuned
upside down.

The object is now ready for esaminatioQ with ■
ttimple lens or with a compound microscope of lo«
oliJBctive. It is by £ir the best for the studeut ID
begin at least with the simple lens. Id order iJut
everythiug may be seen at iLa best, the slide sbonld
be kept warmed to about 38", by being placed on ■
hot stage.

D. Surface vic.u of ike transparent €mbry«
from above.

The chief points to l« obserTed are :

1. The fieadfoU.

2. Ike m^^!CB.^kn« oC *ha amnion; csjiecially tk

APP.]

80RFACE VIEW.

The neural tube : the line of eualeacence of the
meJullary folds, theirs* eernbral vegich, the com-
mencing oplie vemclea, the indications of the
*Kond and third cerebral vesicles, the an yet open
medullaty folJa at the tail end.

The AnariBOBD dimly through tlie neural tnl>e; note
ita pulsation if prescut.

The fuld of the lomilopleure anterior t« the heart
(generally very feintly ahewii).

The /old of the iplanrhmplf^tri (more distinctly
Been) : the vitelline veins.
The megobtoitxc somites.
Indications of the viteUim arleriti.
The as yet barely formed tail-fold.
The commencing blood-vesKla in the pellucid and
TUBCiilar areas.

Surface wVrc of the transparent embryo from
below.

The ooTersIip muat now be removnil and tlie glam
slide again iuniened in a vcs-<el of clean f»lt hoIu-
tion. By gently seizing the extreme ed|re of thi-
opaque area with a pair of forcepis no difficulty will
be found in bo floating the blastoderm, as to turn it
npaidedowD, and thus (o replace it on the sGde with
the under anrface nppcrmosL

The points which mort deacrre attention io thin

Thp hetat: its potition, its u
reins, ita arterial cmL

n with tlie vitelline

PRACTICAL DIBICTIONS. [Ut.

i. The /old of the tpianehnopUure marking the tind
limit of the gut ; the vitelline Teios ruuniog lieaf
its winga

1. The mesohtattie aomiUt on each side of the nennl
canal behind the heart; farther back still, the rvr-
tebral plates not divided into eomitas,

T/:£ examination of the embryo as an opaqm
object.

Thia should never be omitted. Many poicto in
the transparent enihrjo only become intelligible afttr
the examination of it as an opaque object.

Having removed the putty ring and coveidif^ if
previously used, allow the blastoderm bo far to ^
come dry, that Its edge adheres to the glau lUdb
Care must of course be taken that the embm iH^
. does not become at all dry. Place the glaat dids
with the hlastodemi extended flat on it, in a ehftllo*
vessel containing a solution of picric acid (I. B.).

If the blastoderm be simply immersed by itwlf >>>
the picric acid solution, the edges of the optqiu
area will curl up and hide much of the embryo. Tti*
method suggested above prevents these incoavBU-
ences.

The embryo thus hardened and rendered opaque
by immersion in the acid (a stay of 2 to 3 honrB in
the solution will be suRicient) may be removed to n
watch-glass, c-ontaining either some of the solution, oi
plain water, and examined with a simple lene, nmlrr
a strong direct light. The compound niicrosoope will
Ite found not nearly so advantageous for this pur|ioie
as the simple lens. A piece of black paper plaMd
under the watch-glaas, will throw up the Ughta and

APP.] SURFACE VIEW. 448

shadows of the embryo, with benefit. The watch>
glass should have a flat bottom; or a shallow flat
glass cell should be used instead.

a. Looking at the embryo from above, observe : —

1. The head-fold ; the head distinctly projecting from
the plane of the blastoderm, and formed chiefly by
the forebrain and optic vesicles.

2. The elevation of the medullary canal, and the
indications of the side walls of the embryo.

3. The indications of the toil.

4. The Amnion partly covering the head. Tear it
open with needles. Observe its two folds.

6. Having turned the blastoderm upside down,
observe the following points, looking at the embryo
from below.

1. The hinder limit of the splanchnopleure in the
head-fold, marking the hind limits of the fore-
gtU. The opaque folds now conceal the head almost
entirely from view.

2. The commencing tail-fold, and the shallow boat-
shaped cavity (of the alimentary canal) between it
and the head-fold.

The student should not fail to make sketches
of the embryo, both as a transparent, and as an
opaque object^ seen from below as well as from
above. These sketches will be of great service to
him when he comes to study the sections of the
same embryo.

■tractical DmEcnoSl:

The following transverse sections will perhaps it
the most instructive.
Muuipulation as in t. B. 3.

1. Through tlie optic vesicles, sheiriiig the oiitio
stulks.

2. Through tLo Lind-brain, shewing the auiiitofj

3. Tlii-ough the middle of the heart, ehewioj; it« n-
ktiona to the splunchnopleure und alimeatarj' caii»!-

4. Through the point of divergence of the sptancli*
nopleure folda, shewing the venous roots of tha
heart,

5. Through the dorsal region, shewing the medolisir
canal, mcsoblaatic somites and commencing cImvo^
of the mesoblast.

G. Through a point where the medullar}' uui&l is still
open, shewing the mode in vhich its cloidug takci
plaoo.

Longitudinal sections should also 1*e made anil
compared with the tranaveraa Bectiona.

1. Examination of an Embiyo of abont 48 — 60 honn.

A. Opening the egg — as in II. A.

B. Examination of the blastoderm in situ.
Observe

1. Thoybrm of thp embri/o, which b much more dis-
tinct than at the earliiT stjigu.

2. The beating of the lieart.

3. The general features of the circttlalion.

APP.]

TRASSPAltEST EMBRVO.

C. Removal of t/ie Embryo from tlu yolk, as in
II. C.

D. Surface view of the transparent cmbtyo from

\. General fonn of'lJie embryo.

a. CoinDienciug eranial jUxnri:.

b. TLe tail and sidefoldn.

2. Amnion. Notice the imier aiid outer (false nmoioii)
limba and remove them with a needle. When the
amnion has been removed the features of the
embryo will bo much more ulearly Tiaible,

3. The ort/ann o/tejue.

a. JSj/e. Formation of the Uits aji'-mdy nearly
completed.

b, Auditory involiitioii, now a. deep bilu u'ilh ii
narrow opening to the exterior.

i. The braiTU

a. The veaicles of the fore-, mid-, and Aint^faruiii.

b. The ca-ehral vfsicle.

c. The cranial flexure taking jilaco at the mJd-

E. Tratisparent embryo from below.
Manipulation as in IL £.
Notice : —

I. The increaae of the Iv^lfoldi of the Bomatoplenre"
and splonchnopleur'', eHpeeialiy the latter, and tin-
conuueuijement of these fukla at the tail

PRACTICAL DmECTION'S. [irp.

2. The now cc-shaped luiart; for furtUer pftrtieulnM
vide Chap. iv.

3. The oommencing 1st oud 2nd vUeeral cUJU tmi
the aorlie arehea.

4. Thectrcuiu:w»o/i/M!!yoZigac,videFig. 36. Mika
out all the points there shewn and Rseertua
by examination that what hftva been called the
veins and arterlea in that figure, are truly eduL

T/ie embryo as an opaque object,
Ti-eatnient aa in II. F.
Frd» above :

Observe the amnion^ vrhich is a very conspicoom
object, and remove it vith needles if not done pre-
viously. The external form of the brain aud tlie
audUory tac appear very distinctly.
From below ;

Observe the nature of the h«ad- and laUfMt,
which are much more easily understood from die
opai^ue than frem the ti'ansporent embrj'os.

Observe also the alim^idari/ canal, the widel*
Open hind end of the fore-gtU, and the fnml end of
the as yet very short htnd-^.

i. Sec/ ions.

Manipulation as in L B. 3.
The more important sections to be observed, a/c
1, Through optic lobea, shewing;
a. The formation of the lens.
h. The involution of the primary optic vesicle:
0. T\ie cuTiuU-ictiou, «s^cially from above, of tlw

THIRD DAY EMBRYO. 4+7

2. Through auditory sac, shewing ;
a. Auditory aac still open.

6. The thin roof and thick sides of the hiQd-brain.
e. Notochoi-d.

d. Heart

e. Closed alimentary canal

3. Through doi-aat region, ehewing the general appear-
ance of a section of an enibiyo nt this stage, which
should lie compared with a similar section of tho
earlier stuge.

It shews :
a. The commencement of the side folds ', the ali-
mentary canal atill however open below.
6. The Wolffian duct lying close under the epiblast

on the outside of the mesoblastic somites.
e. Tlie notochord with the aortie on each sida

Eiamination of an Embryo a,t the end of the third
day,

A. Opening the egg, as in II. A.

B. Examination of the blastoderm in situ.
Observe : —

I, The great increase of the viuevlar area both in size
and distinctness. Tlie circulation is now better
seen in titit than aft^r the blastoderm has been
removed.

1. That the embryo non lies completely on its lijl
tide and that it is only cooQected with the yolk-sac
hy a somewhat broad alalk.

Removal of tlu embryo. See 1 1. C.

It is now unnecessary to remove tlie whole of tJie
blastoderm with tbe embryo ; indeed it is bottpr to
cut &w»y tlie vascular area unless it ia v'&nt«d fur

Surface i-iew of the transparent embryo.

Since the embrro now lies on it^ side vc slmll
not hnve to speak of the view from above aod below.
The views from the two sides differ chiefly as to Uw
appearance of the heart.

The embryo (freed from the blantodcrm and ihc
amnion) ia to be floated ou to a glaaS slide in the
usual way. It ia neceasary to protect it while nndtr
examination, with a coverslip, which must not be
Etilowed to compress it Toavoidthia, we have found
it a good plan to support the corerstip at one end
(inly, Hiace by moving it about when thus supported,
a greater or less amount of pressure can b« applieil
at will to the object.

Tbe detuila which can at this atago bo seen in ft
tmnspareut embryo are very numerous and we re-
commend the student to try and verify everything
shewn iu Fig. 37. Amongst the more importAntand
obvious points to be noticed are

I. The increase of tlie eraniat Jlexure and the body-

JleXUTS.

I. The condition of the brain. The midbrain now
forma the most anterior point of the head.

The fore-brain oonsisbi of tlie iiiconspicuoiu
veaide o? U\e tkicd venti'icle and the two liryi
cerebisX \o\ie&.

p.] OPAQUE EMBRYO, 449

The hind-brain coiisiBts of a front portion, tlie
cerobellum with a tbickenud roof; and a hinder
portion, the fourth ventricle with a very thin and
delicate roof,

3. Oryant of sense.

The «ys especially is now in a very good state
to observe. The student may refer to Fig. 51,
Mid the duBcription there given.

The ear-vesicle will be seen either just closing
or completely closed.

4. la the region of the heart attention must also \x

a. The vUceral cleJU.

b. The inveaiiig-mase, Le. the growth of meaoblast
taking plaoe around the end of the notochon).

e. The condition of the hearL
6. In the region of the body the chief points to be
observed are ; J

a. The increase in the number of the eomiles. H

b. The Wo/Jian due'., which can be seen as a streslrl
along the outer side of the hinder somites.

e. The allaiUoU, which is now a smidl vesicle lying
between the folds of the somatopleure and
splanchnopleure at the hind end of the body, but
as yet hardly projects beyond the body cavity.

E. TAe embryo as an opaque object.
Preparation as in II, F.

The general form of the embryo can be very satis'
factorily seen when it is hardened and examined as on
oj>aque object; but the moa*. uiv^'ttaiA \wMi'^a '^

PRACTICAL DIRECTIONS. [AP?.

mftde out at this etage in the hardened specimena an
those connected with the visceral clefts and folds uui
the mouth.

If the amnion has not been removed it will he
necessary to pick it completely away with neeJlea.
Without further preparation a view of the viscerJ
folds and clefts may be obtained from the side ; hot
a fiir more instmctiTe view ia that from below, ia
order to gain which tlio following method may ba
adopted.

Four a small quantity of melted black wax (mails
by mixing together lampblack and melted wax) into
a watch-glass, using just enough to cover the bottom
of the glass. While still soft make a small depremits
in the wax with the rounded end of a pen-holder <x
handle of a paint-brush and allow the wax to cooL
In the meautiue cut off the head of the haidened
embryo by a sharp clean transverse inclaion canioi
just behind tbe visceral clefts, transfer it to tlie
watch-glass and cover it with water or spirit. By
tittle manipulation the head of the embryo may no*
be shifted into the small depression in the wax,
and thus be made to assume any required positioa
It should then be examined with a simple
under a strong reflected light, and a dravring
of it.

When the head ia placed in the proper podticov
the following points may easily be seen.

1. The opening of tfte tnouth bounded below bytl

Jirat pair of visceral faldi, and commencing to '

ouclosed above by the now very small buds wbi

RTo \,W TiiSQaiiiHa qI t\uj sa^ienor tnaiillary ^

ceases. ^;«TO-?Nt%'^vi. ^^.

,APP.] FOOETH DAT EMBBTO. 451

3. The teeoni and t/tird visceral arches and cle/it.
3. The nasal pits.
F. Sections, Manipulation as in I. B, 3.
The moat important sections are : —

1. Through the eyes in the three planes, vide Fig. 50,
A. B. C.

2. Through the auditory sac.

3. Through the dorsal region, shewing the general
changes which have taken place.

Amongst these, notice

a. The changes of tke vusoUaslie sojnilea: the com-
mencing formation of the muscle-plates.

b. The jwsition of the Wolffian duct and the forma-
tion of the germinal epithelium.

c. The aorUe aad the cardinal veins.

d. The great increase in depth and relative diminu-
tion in breadth e£ the section.

V. ExMainatloii of an £mbi;o of tlio Fomtli Day.

A. Opening tite egg, as in II. A.

Great care vrill be required not to injure the
embryo, which now lies close to the Bhell-membrane.

B. Examination in situ. Observe; —

1. The now ooiiBpiciioua amnioa.

2. The aUmUois, a small, and as yet hardly Tssoutar
veuicle, beginning to project from the embryo into
the space between the true aud the false a

3. The rapidly narrowing somotVc itoUt.

tSa PRACTICAL DIRECTIONS. [APP,

C. Removal of the embryo, as in II. C, and IV. C

The remarks made In the latter place apply vitJi
still greater force to aa embiyo of the fourth and
eucceediDg days.

D. Surface view of the transparent embryo. For
manipulation, vide IV. D.

The points to be obserred are : —

1. The formation of the fjih, ^oeaO\, and nituh
eranial nervea.

To observe these, a small amount of pressure
IB advantageous,

2. The formation of the fourth mtceral cleft, luid the
increase in size of the superior mfucillanr prDoeaa.

3. The formation of the nasal pits and grooveg.

i. The great relative growth of the cerebral lobea and
the formatiou of the pijieal gland from the roof of
the vesicle of the third ventricle.

5. The great increase in the invettuvj mast.

6. The foimatloa and growth of the mttscle^lales,
which can now be easily seen from the exterior.

7. The altantoig. Make out its position and mode of
opening into the alimentary canal.

E. 7"Aff embryo as an opaque object. Manipulation
as II. F. For mode of examination vid*
IV. E.

The view of the mouth &om underneath, shewing
the naaal pit and grooves, the superior and inferior
maxillary processes and the other visceral folds and
clefts, ia very instructive at this stage. Compare
Fig. 69.

IPP.]

TWENTY HOURS EUBIirO.

F. Sections. Manipulation as in I. B. 3.

The most importtuit sectiona are,
1. TliTougb the eye*.
3. TranByerae section immediately behind the risoeral

fttches, shewing the origin of the lungs.
3. TranBverae section just in front of the umbilical

stalk, showing the origin oE the ft'ocr.
i. TransTerse section at about the centre of the

dorsal region, to shew the geTieral /eatuTts of the

fourth day. Compare Fig. 68.
Amongst the points to bo noticed In thia section, are

a. Muscle-plates,

b. Spinal nerves and ganglia,
C; Wolffian duct and bodies.

d. Mailer's duct.

e. Mesentery.

y. Commencing changes in the spinal cord.
6. Section passing through the opening of the allan-
tois into the alimentary canaL

For the points to be observed in embryos of
the fifth and sixth d&jB, the student must consult
the chapters devoted to those duys.

In the hardened specimens, especial attention
should be paid to the changes which take place in
the parts forming the boundaries of the mouth.

.tion of a Blastodeim of 20 honn.
Opening tlie egg, as in II. A.
Examination in situ.

It will not be found possible to mnke out an'
very satisfactory from the exwja.uBSio'v ^

anythln^^^^H

LM PRACTICAL DIEECnoNS. [APT.

derm in situ at this nge. The BtuJent will liowerer
not fail to notice the halones, whicli can be snu
forming concentric rings round the blastoderm.

C, Removal of tJi£ embryo.

Two methods of hardening can be adopted >t
thia age. One of theee involves the removal of the
blastoderm from the jolk, as in IL C, In the other
case, the yolk ia hardened as a whole. If the latter
method be employed, the embryo cannot be viewed
aa a tranaparent object.

In tbo cases where the blastoderm is TemoTed
from the yolk, the manipulation ia similar to th»t
described under II. C, with the exception of more
care being required in ireeing the blastoderm bom
the vit«lline membrane,

D, Surface view transparent, from above.
Observe: —

1. The medtdlarr/ groove between the two merluUan/
folds, whose hind ends diverge to enclose between

them the end of the primitive groove.

2. The headfaid at the end of the medullary groove.

3. The one or two pairs of mesoblastia tomites flanking
the medullary groova

4. The nolodiord as an opaque streak along the floor
of the medullary groove.

E, Surface view transparent, from below.

&amtt '^Wa \jQ \i« wan as from above^ bat Its
dettcV'j.

APP.]

TWENTY HOURS EMBRYO,

435

F, Embryo as an opaque object.

As an opaque object, whether the embryo ia hard-
ened in titu or after being removed from the yolk,
the same points are to be seen aa whea it ia viewed
as a ti-unsjiarent object, with the exception of the
notvchord and mesohlastic gomites (vide D). The
various grooves and folda are liowever seen with far
greater clearness,

G. Sec lions.

Two methodfl of hardening may be employed ;
(1) with the embryo in nlu, (3) after it has been
removed.

To harden the blastoderm t» situ the yolk must
be hardened as a whole. After opening the egg either
leave the yolk in the egg-shell or pour it out into a
Berlin capsule ; in any case freeing it as much ss
possible from the white, and taking especial care to
remove the more adherent layer of white which im-
mediately enrroiinds the yolk.

Place it in picric acid or a weak solution of cbromic
acid (first of 1 p.c and then of "5 p.c.) with the
blastoderm uppermost and leave it in that position
for two or three days.

Care must be taken that the yolk does not roll
about; the blastoderm must not be allowed to alter
ita position : otherwise it may be hard to find it when
everything has become opaque. If at the end of the
tecond day the blastoderm is not sufHciently hard,
the strength of the solution, if chromic acid be used,
should be increased and the specimen left in it for
another day.

After it has beconie hardened by the acid, the
/oli should bo washed -wilk -w^Vm »a!^ ^KwAe^L-wier

PRACTICAL DmECnONS. [aW.

ceasivel^ with weak and strong spirit, vidi L K
After it has been in the strong spirit (90 p.c) for tn
days, the vitelline membraDe may be aafelj peeled off
and the blastoderm and embryo will be found »
tilti. The portion of the yolk containing them nmat
then be sliced off with a sharp razor, and placed m
absolute alcohoL

The staining, &q. may be effected in the ordiiui;

If oaiuic acid, which we believe will bo founil
flerviceablo for these early stages, is employed, it will
lie necessary to remove the blastoderm fivm the yolk
before treating it with the reagent.

The following transverse sections arc the most im-
portant at thia stage :
I. Through the medullary groove, shewing
o. The tnedtUlary /oidi with the thickened meeo-

blast.
b. The notoehord under the medullary groove.
e. The commencing cleavage of the mesoblatL
i, Xhrougb the region where the mednllsry folds
diverge, to enclose the end of the primitive groove,
shewing the greatly increased width of the medul-
lary groove, but otherwise no real altentioo in
the arrangement of the parts.
J. Through iho front end of the primitive groove
with the so-called axis cord underneath it, whils
on each side of it are still to be seen the medul-
lary folds.
I. Through the primitive groove behind this poin^
a\i«wm% *^« Vj^\caV c&u»ftteta of the primitin
groove.

457

•P.] DNtSCUBATED BLASTODERM.

fn. ZSzamltution of an anincnbated filastodarm.

Opmifig the egg. Vide 11. A.
Examination ofi/te blastodtnn in situ.

Observe the central white spot and tbe peripheral

more transparent portii
halones around it.

1 of the blastoderm and tho

C. Removal of l/te blastoderm. Vide VI. C.

With tho unincubated bJaatoderm still greater care
is required in removal than with the SO hours' bliiato-
derni, and there is no speciul advantage in doing so
uolesa it is intended to harden it with osmic acid.

D. Surface view transparent from above.
Obserre tho absence of the central opacity.

E. Surface view transparent from underneath.
Nothing further to be observed than from above,

F. As an opaque object.

There ia nothing to Ije learnt from this.

G. Sections,
Manipulation as in YL 0.
The sections shew

a. The distinct epiblast.

b. The lower layer ceils not as yet differentiatft
into niesoblast and hypoblast.

e. The thickened edge of the blastoderm.

d Tbe aeffnieiiUUion cavity aui/ormoi^'c* tdl*. ,

43S PRACTICAL DIKECTIOSS. [jlPF.

VIIX Examination of tlie process of Segmentation.

To observe the process of eegmentiition it will Ic
found necessary to kill a number of hens which an
laying regularly. The best hens lay once every 31
hours, and by observing the time tbey usuiilly lay (ui<l
they generally Iny pretty regularly about the oune
time), a fkir guess may be made beforehand as t«
the time the egg has been in the oviduct. By this
means a series of egga at the various stages of wg-
mentation may usually be obtained without a gr^ai
unnecessary BacriUce of hens. For making secdoiu,
the yolk must iu all cases be hardened as a whoU,
which may be done as recommended in TL G.
Chromic acid ia an excellent reagent for ihU tnJ
it will be found very easy to mnke good sections.

In the sections especial attention shoold be paid,

1. To the first appearance of nuclei in the segmenii.
and their character,

2. To the appearance of the liorizontal furrows.

3. As to whether new segments continue to be fijnned
outside the limits of the germinal disc, or whether
the fresh segmentation merely concerns the already
formed aegraenta

i. Iu the later stages, to the smaller central and
larger peripheral segments, both oontalning nnclel
For surface views, the germinal disc, either
fresh or after it has been hardened, can be used.
In both cases it should be examined by a atmo;
reflected light. The chief point to bo noticed is
t\iQ TUQTft Tn.^W «i^gic«D.\a^v3Q. Qf the ccutrsl than of

(APP.] STUDT OF BLOOD-VESSELS, iSd

!IX. Ezaminatioii of tho later changes of the Embryo.

For tbe tater stages, and especially for the deve-
lopment of the akull aiid the vascular system of the
body of the chick, it will l>e found necessary to dissect
the embryo. This can be done either with the fresh
embryo or more advantageously with embi'yos which
have been preserved in spirit.

If the embryos are placed while still living into
Hpirit a, natural injection may be obtained. And such
nn injection ia the best for following out the arrange-
ment of the blood- veasela.

Sections of courae will be available for study,
eapecially when combined with disaectious. ■

X study of the development of the Blood-veasels. ^

Observationa on tJiis subject must be made with
bUatoderma of between 30 — 40 houra. These are to
be removed from the egg, in the usual way (vide IL
A. and C), spread out over a glass slip and examined
from below, vide IL E.

The blaatoderro when under examination must be
protected by a coverslip with the usual precautions
against pressure and evaporation, and a hot atagR
must alao bo employed.

Fresh objects so prepared require to be examined
with a considerable magnifying power (400 to 800
diameters). Prom a series of specimens between 30
and 40 houra old all the points we have mentioned
in Chapter iv. p, 92, can without much difficulty be
observed.

Especial attention should be paid in the earlier
specimens to the masses of nuclei enveloped in pro-
toplsBm and connected -wVlii twiV u'l^'ct \(^ -^p^Aiir.

PRACTICAL DmEcnoss. [aw.

plasmic processes; and in the Inter stages to tlu
breaking up of those masses into blood corpiuda
ajid the conversion of the protoplasmio processes
into capillaries, with cellular walls.

Blastoderms treated in the following wbjb msy
be used to corroborate tlic observations mode on Uie

With gdd ddoride.

Immerse the blRstodenn in gold chloride ('5 pc)

for one minute and then vash with distilled water

and mount in glycerine and examine.

By this method of preparation, the nuclei snd

protoplasmic processes are rendered more distinci,

without the whole being rendered too opaque for

observation.

The blastoderm after the appHcation of the gold

chloride should become a. pnie straw colour; if it

l>ecomes iu the least purple, the reagent has been

applied for too long a time.
With potaaeium bickromaU.

Immerse in a I p.c. solution for one daj and than

mount in glycerine.
With osmic aeid.

Immerse in a 5 p.c Bolution for half an hour ami

then in absolute alcohol for a day, and fimdly maunt

in glycerine.

PrACTICAI. DlBECTIOXa FOR OBTAINIKO AND STUDTtSO
UAUHALIAN EuBBYOS.

lI. AntmalB and lireedln^.

For claRs work the Babbit ia the moat ooaTOuait
auiiDa\ ^tom. 'vbich to obtain embryos, it will lined

APP.] MAMMALIAK SEGMESTING OVA. 461 I

freely in the early spring moatlis of the year and will
gire ample opportunity for the student to observe the
exact time when the fomale is covered, A number
of doee should be kept together in a large pen, and
two or three bucks in separate small cages also placed
within the pen ; at the period of heat, the doe should
be temporarily placed with the buck and tlie exact
time of copulaCioD noted, the age of the embryo
being calculated from that hour.

Examinatioa of Bsgmentiiif ova.

It M-ill be well to mention here that although
a doe may have been satisfactorily covered, embryoe
are not always obtained from her. A superficial
esamination of the ovaries will determine whether or
no fertilized ova are present. If ova have been
recently dehisced from the ovary, the Graafian follicles
from which they were discharged will be found to be
of a distinctly red colour. In case no snch ' corpora
lutea ' as they are called are present further search ia
uselesa.

To obtain ova from i to 60 hours old.

Cut open the objomen from pubis to sternum,
and from the pubis round the thigh of each aide, and
turn baclc the flaps of the body wall bo formed.
Remove the viscera and observe below (dorsal) the
nngle median vagina, from the anterior end of which
the uterine horns diverge^

Observe at the anterior end of each uterine horn
ft sniall much coiled tulie, the oviduct (Fallopian
tube) near the anterior end of which a httle below
the kidney lies the ovary. Cut out the uterus a
ovidnot together and lay them in a small dissectiiifr

62 PRACTICAL DIRECTIONS. [app.

(iisli. Carefully stretch out the oviduct by cutting
the titwue which binds it, and Btiparating it from
the uterus, taking care to obtain ita whole length,
lay it upon a gloas slide.

With the aid of & lens it is frequently possible to
distingui^ the ovum or ov&, through the wall of tlie
oviduct. In this case cut a transverse Blit into the
lumen of the duct with a fine pair of scissors a little
to one side of an ovum ; press with a needle upon
the oviduct on the other side of the ovum, which wiU
glide out throuj-h the slit, and can be with ease trans-
ported upon the point of a sniall scalpel, or what is
better spear-Leaded needla la case the ovum cannot
be distinguished in the oviduct by superficial obser-
vation, the latter must be slit up with a line pair of
scissors, when it will easily be seen with the aid of an
ordinal? diaeecting lens.

B. Treatment of the ovum.

The ovum maybe examined fresh in Bait solution,
it is however more instructive when preserved and,
stained in tlie following manner.

a. Immerse it in a J p.c. solution of osmic acid for
5 or even 10 minutes, transfer it thence to
the picrocannine solution described above (I).
After staining the ovum should then be washed
in distilled water and placed iu a 'weak solu-
tion of glycerine in a watch-glass — half gly.
cerine, half water. It should be allowed to
remain thus under a bell jar for several days
(7 to 14 or longer) iu a warm room until the
water has e\*aporated. By this means shrinkage
aQiOiistQT\.\i«iM«wQided, the glycerine becomlug

EXAMINATION OP OVUM. 463

very gradually more and more dsnae. It should
be mounted io glycerine m which 1 p.c. formic
acid has been mi:(ed to prevent fungoid growths.
Care nmst be taken that there ia no pressure
upon the ovum this being insured by the inser-
tion of a. couple of slips of jmper one on each side
of the ovum under tho cover glass.
b. Another method of preservation ia used, but
does not appear to us so successful c
already described. It consists of an i:
of the ovum for 5 minutes in J to J p.c. oamjc
acid, aubaequent treatment with Mailer's fluid
for two or three days, and finally mounting in
glycerine.

C. Exatniftadon of the ovum.

The moat instructive stages to observe are ova of
a. 18 houra old, when four segmentation spheres

will be observed.
h. 36 hours old when the segmentation ia more
advanced and the spheres numerous.
The chief points to be noted are ; —
The number and size of tho aegmontation spheres ;
in each of which, when treated as described in B. a.,
a targe d(!e]>ly stained nucleus will be visible. The
spheres themselves are also atained slightly.
Tho presence of one or two jjoW bodiea on the
oiiUjr aide of the aegmenta in ova of not more tbaa
4$ hours old: these also are slightly stained.
The eona radiata immediately surrounding the
segments, and

The thick albuminous coat, marked with
centrio rings.

IM PRACTICAL DIRECTIOKS. [APP.

D. The fully segmented ovum. 70 hours old.

The fully segmented ovum is found in the uteriu
at its anterior end close to the place vhore the
oviduct opens into the uterus.

To uhtuin this stage the uterus must be slit open
and examined carefully with a dissecting lens : the
ovum will be seeu as a somewhat opaque spot on the
glistening moist mucous epithelium of the utenis.

It may he treated in the manner described under
B. a,, but the segmenta being closely pressed to-
gether their outlines are not rendered distinct by
this method. A more advantageous mode of treatment
is the following : wash the ovum rapidly in distilled
water, and place it in a 1 p.c solution of silver
nitrate for about 3 minutes : then expose it to the
light in a dish of distilled water until it be tinged
a brown colour.

The brown colour is due to the reduction of the
silver, which takes place chiefly in the cement eub-
Btance between the cells and thus defines very exactly
their sise and shape. The ovum may now be treated
with glycerine and mounted aa described in B.

The points to be observed are : —

1. The division of the segmentation spheres into the
layers — an outer layer of cubical hyaline cells, and
an inner of rounded granular cells,

2. The blastoiwra of van Beneden.

3. The presence of a thin layer of mucous outude
the concentrically ringed albuminous coat of the
ovum.

XTI I. Examination of the blastodermic Tealcle, 72—

A. To obtain the embryo see XII. D.

B. Prepare the ovum eit/ier as in XII. B. .
or in picric acid see\.1A. i.

C. Surface viczf, or in section see I. B. 3.

1, The great increaae in size of tlie ovum and the
reduction in the thickness of the mcmliranet).

3. The flattened Uyev of outer cuUb euciosiiig a cavity.

3. The rounded cells of the innei' mHea attached aa a
lutut-ahaped masa to one side of the vesicle.

, Examination of a blaatodeimic vesicle of 7 dars,
in which the embryonic area and primitive streak ate
present.

To obtain tlw embryo.

On opening the body cavity the utenia will be
found to be uniformly swollen and very vascular.

Remove the utenia and o|wn it carefully with
tine Bcissors along the free, non-mosometric edge,
taking care to keep the point of the scissors within
the uterus close against its wall.
Observe
I. The oval tbin-walled vesicles lying at intervals

on the walls of the uterus.
I. The presence of the jjyriform embryonic area, at
the posterior cod of which is seen the primi'

466' PRACTICAL DIRECTIONS. [Ut.

3. The commencement of tlie area v&sculosa »io«nd
the hind end of the area. This is seen 1«tter
after treatment with picric acid.

B. Treatnunt and Examination of the embryo.

a. Preaerve the veeiule in picric aee 1 B. 1.
Stain in haematoxjlin, cut out the embryonic
area, leaving a conndorable margin, imbed and
cut into sectiona

6. In trana\'erae aectiona observe : —

1. At the anterior end of the area, the single row of
columnar epiblast and the single row of flattened
hypoblast cells.

2. Immediately iu front of the primitive streak be-
tween these two layers a few irregtUarly shaped
mesoblast celts.

3. Through the middle of the primitive streak,

a. Several layers of roundi^ mesoblast cellsattacbed

to, and continuouB with, the epiblaat in tlift

middle line, and stretching out laterally beyond

the edge of the area.

h. A single layer of flattened hypoblast.

i. The epiblast outside the embryonic area

form of flattened cells and, except in the

around the primitive streak, overlying a layer of

flattened hypoblast

XV. Examination of an eight days' embiyo.
A. To obtain Hie cfnbryo.

The uterus will 1<e found here and there to It
swollen. In these swellings the embryos lie'

'.] EIGHT DAVa' EMBKVU. ■ 467

owing to the fact that the wall of the embryonic
Tesicle is exceedingly thin, and attached to the ute-
rine wall, they are very difficult to obtain whola

Cut the uterUB trans ve we ly on each aide of the
swellings and pin the pJecea ho obtained slightly
stretched out in small dissecting dishes. Cover the
tissue with jticric acid solution and allow it to remain
untouched for an hour. Then with two pairs of fine
pointed forceps carefully tear the uterus longitu-
dinally, slightly to one side of the median Hue of the
free side. This operation will necessarily take some
time, for but a small portion should be done at once,
the picric acid being allowed time to penetrate into
that part of the uterus witich has been most recently
torn open.

With care, however, the student will be able to
open completely the swelling and will obiierve within
the thin walled vesicle. Great cure must also be
exercised in freeing the vesicle from the uterus.

This dissection should be performed with the aid
of a dissecting lens. In case the embryonic vesich'
ia bnrst it will still bo possible to extract the embryonic
area which lies on die mesometric side of the uterus;
the aretk itself is not attached to the uterine walls.

1. Examitiation of surface vlev.'.
Observe ;

1. The increased size of the embryonic ai-ea.

2, In the anterior region the medullary fohb; j
verging behind and enclosing between them,

;(. The primitive streak.

4. Tlie area optica now completely euirou

PRACTICAL DIHECTI0K8.

(APP.

C. Examination of sections.

Piv|jBre and cut into transverse sections aa advisnl

in XIV. E.
Notice

1. In the sections of the anterior re^on,

iL The lateral epiblaat comjioseil of aevenil layen

of columnar cells.
U. The epibUst iii the mtHiiun line one layer thick

and in the form of a gi-oove (medullary groove).
f. The lateral plates of meaoldast.
d. The flattened lateral hyjKihlast, and cotunuuLr

hypoblast undci'lyiug medullary groore (noto-

chord).

2. In sections through the anterior end of the priini-
tive streak.

Note the continuation of the epiblast, mesobkn
and hypoblast in the middle line.

3. In sections through the posterior eud of the area
the some jxiinte to he sE«n as in XIV. B. h. 3.

XV1. Examination of an embryo about 8 days 12 hoars.
A. Manipnlaiion as in XV. A.
H. In surface view observe (cf. Fig, 106) :

1. Area pellucida sumjunding enibryo, outside which

is the well marked area vaRculosa.
% Widely open neural canal, at anterior end dilated,
and partially divided into the three piitnary ven-
cles of the brain : noie the optic vesicles. At the
posterior end, the ainiw rhomboidalis.
The meeoblaatic somites, 4 to 8.

The two Literal tul>c« of the hi-arl, aiiil th« onm-
moncement of the two vit«lliiit> vcinft,

The rudimont of the primitivp stieok,

The commencing head ftnd tail folds.

The comnieucing folds of the amaion.

Comi>aro Fig. 106.

XVIL Eiaminatioit of the fcetal membranes of an embrjo
of 14 days.

A. To obtain the embryo, with its membranes.

Manipulato as in XV. A. oolj" liisseft umler null
solution instead of picric acid.

B. Obsen'c before remin'iuff the embryo from tlit

litems :

1. The attachment of the vesicle to the miwomntrio
tdde of the uterus over a discoidal area, the
placental areo-

2. The position and foim of the plnccnta.

C. Remove t/te embryo with its membranes intael,

and observe :

1. the TsacaUr jollc lac, extending compl«t«1r nximl
tbe choriim witb the exoeptioa of m oom|i»rativ^7
muUl arm where

•i. the ftllantois is situatexL Tbe ratcuUritj ot ihf
allaolMs. The bHal rilU pnjwting into tkm

470 PRACTICAL DIRECTIONS. [aPP.

D. Separate t/te membranes from one another with-
out tearing them,

and notice :

1. The embryo surrounded by the amnion.

2. The allantois ; its position dorsal to the embryo; its
attachment to the chorion ; its circulation.

3. The flattened yolk sac, ventral to the embryo ; its
long stalk ; its circulation.

4. The heart

E. The embryo in surface view.
The points to be observed are

1. The cranial and body flexure, the spiral curvature
of the hinder portion of the body.

2. The vesicles of the brain : cerebral hemispheres,
fore-brain, mid-brain and hind-brain.

3. The eye, and the ear.

4. The heart.

5. The visceral arches and clefts.

6. The fore and hind Hmbs, and the tail.

APP.]

Note A.

Siitce writing tbe account of st-ction-cutting on p. 43-1,
I we have obtained more exjierience as to the practical work-
tng of Messrs. Caldwell and Threlfall's microtome there
tn<^ticned. We find that it cuts more accurately and better
than auy other microtomu with which we ai-e acquainted,
and can confidently recommend it to inTestigators and
teachers with large classes. In tJie Cambridge Laboratory,
it is driven by a small water engine and wOl cut at a rate
of 5(X1 a minute, without detriment to the sections.

Note K

Mr ThrelfaJI, of Caias College, has recently elaborated
a method of mounting sections which in our opinion has
many importtuit advantages over the shellac method. Tt im
as follows. Make a solution of pure india-rubber in benzine
or chloroform. Spread a thin film of this on a clean glass
slide, and allow it to dry. Arrange the sections on thf>
film ; melt the paraffin ; allow the slide to cool, then
immerse the slide for a moment in lienzoUne (liquid
paraffin), which dissolves tlie [larafiin, and mount in balsam.
The chief ndvautages of this method are that the sectiona
do not ndhew to the india-rubber until warmed, aiid they can
be stained after tbey are fixed on the slide if necessary.
For the latter purpose, wash the lienzoline uway willi
absolute alcohol ; treat with weaker alcohol ; stain ; return
to absolute; clear with oil of cloves or kreasote, rmd mount
in lialsam {vide Zoohgiteher Aifiei'jer, 18H3).

I ,

Abdominal wall of cUiok, iSi

Air-ohamber, .^

Albnmen : composition of.
kmnuemeiit or, in hen's i^
3 ; [oTDiatioD of, in faen, i
{■te of. in hen's Baa, i oo ;
incubated egg, i

Aluphenoid region of cliick. i^o,

146
Allantoio BFtcries: or cliick. 115,

103, toH: in manunolB, 348,

igo: of mammalB, 34
Allantoic stalk, 351
Allantfoa : of cMak. 1

-33. 46

-+7. '07. 185— I8j. 177. i8o;
u a means of reapiiation, 131 ;
pnlBBttoD of, 177; of rabbit, for-
mation of. 311, .153; of human
embiyo. 336--340. sfs— 358;
otmatuiDalia,Btrn(ilnivof, 3481
ol maraopiaU, 351 ; of dog, 358
Alum cumine, to make and use.
431

63. 107. 195; o'f thirS'daj, j
176 — 180; polsatiou of, 17;.
17S; false, of obiok, 46; of

rabbit, 330. 311.1; of human
umbryo. 3(fi — 340; "f iiinm-

malia, 343; structure of mam-
malian. 346; of dog. 358
Amphioius, spinal cord of. 154
Aunnli fibrosi of birds, no
Anterior comniisaare of cerebral

hemisphere, mammalia, 381
Aorta of cliiok, 114, igi, iq8 ;

89, 103
Aortic arches of ohiok, 103, 106,

167; of fourth day, uj, 191 —

]<)8
Apea' placenta, 355; histolog; of,

363 ; derivation of, 3G4
Aquedurtns veatihali of ohiok,

1.^8
AqneduotUB sjlvii {in Iter.)
AqnoouB htunour; of ohiok, igj —

1 54 : of mammalia. 390
Arbor vitae of binla, 3^
^rea opaca of chick, 7, ^9, 195 1

mesoblBst of, 65 ; hypobUat o(.

65 ; vascular portion of, 74 — 7$,

110; of third day, 109
AteapeUaoida;ofchick. 8. 49. 55;

of thirdday, no; ofniammuls.

3J8

Area vasculosa : of majnmalla,
formation of, 341 ; oirculatiun

Artena uentialis retinffi of iuiiia<

malia, 387— 39»
Arterial ^tem: of chick. 114—

-303; mammalia. 407

-•"(

Anditor; capeole of chiok, J^i
Anilitorj pits of chick, 8:, loi
A-oricleB of chick, 8^, lot, 119,

Auricular: sppendageeofDhickof
second da;, 101 1 septum »l
chick. 357
Avian eharactcriBtics, 375
AxjgoB vein. mammBlia. 411

Blood islands of Tiuculu «m et
ohiok. 91

Blood corp^^'^^ of chick, for'
mation of, Qi — 94

Blood-vessela : of area opaoa 0!
ohiok. formatioD of, gl—g^-.
development of, practical ili-
reolions for fltndyof, 455, 4**

Bod; cavity: of chick. 39; fonn*'
tioD of. 4a, 41 ; jiosUriot medi-
astinum of. 167 ; of mammalia,
406

Body Qeitoe of chick, tg6; im
thirJ day, 116

Body flemre; in rabbit, j^t: in

Bam-hyal chick, 145

Basilar: plate, 3,15 — ijS; tacm-

brane, maromnlia, 397
Basi'Occipilal ret^on of ohiok, 1137
Baai-spheDoid of chick, 340, 146
Baei-temporal bone, chick, 146
Beak ofcbiak, i4g; formation of.

Biliary duota of chick, 180 — iSi

Birds, oviparous, 30S

Bladder : derivation of, in mem-
mala, 351 ; mammalian, 417

BlaEtoderm of chick, 4 ; bIfuc-
taie of, in nnincubated ben's
ei!Hi 7 — 10; area pellncida of,
S; formative cells of. 13, 14;
exteneion of, 16, 17; uteral
folds of, 37 ; head fold of. 17,
37; toil lold of, ig, 37; vas-
cular area of, 17 ; hypoblast
of, 51; germinal wall of, s»'.
epililost, £5 ; of thud day, 109,

Blastoderm of mammal, forma-
tion o[ layers of, 314— 335; vas-
cular area of, 316 ; pellucid
area of, 338 1 bead and tail
folds, 319

Blastodermic vesicle, 314 — 316,
jrg; outer layer of, 314; inner
mass of, 314 ; to examine, 465

Blastopore of

=E. 33*; °f himtna i

itejo.

Bram: of ohiek, 117 — uj, jSii
of mamnjalia, 367 — 387 ; dW-
sionsof, 367: hindbr^in, 36?—
ijo; mid brain. 370. 371 j &»
brain, 371—385 ; nislogeny o^
38S-387
Branchial clefte aad arahcH (im

ViBceral)
Breeding mammals far study. 46a
Bronchi, mammalian, 41B
Bronchial tubes of chide, 177
BulbuBBrterioBUBot chick, 84,1
i3g. 257 ; septum of. 157, s$9,
]6o — -ifii ; of '■

Cncum. mammalia, 419
Canales Botalli {tee Daotu

lalli)
Canaiia anncnlaria of chick,

of chick, 169,

oar of mammalia^ 193 — 398
Cardinal veins : ofoluok, 170',)

— 1S5; anterior and

of niammaha, 409 — 413
Oarraine, 431
Carnivors, placenta of, 358

Ciratid: aoTDmon Brtei7 of chiek.

ig;, ]q8i eiternal and intem&l

ftrtery, igi. igj ; of bird and

mammal, j.d3
Carpus of ohick. 134
CarlUsKe boaee, 341 ; of skull of

ehiok, 146
Cerato-h^als of chicb, 145
Cerebellnm: of chick, ui, 103,

368^370 : of munmalis, ^"7

— 370 ; ventricle of. 368 ; dbo-

loiii pleiuH of, 3fia; pyramidH,

and olivary bodies of, 36B;

fttbor vitiE, flooculi of, 369 ;

pons varolii of. 369. 370; v^nm

medollB ant. 370
Cerebral hemuphar«i : of chick,

117; o( roaionialia. 376 — jSj;

ventricles of, 377; lamina tcr-

minalig. 377; corpus striatom,

378;eommiflfluroao(, 381 — 3B3;

septnm lacidum, 383; lissures

of. 384—385
Cerebral vesicles of chick, aoo;

of second dfiy. 79, 100
Cerebro-Hpinal canal in chick, 40
Cerebnun of mammalia, mono-

tremata, inseulivora, 384
Chalazee, 4

Cheiroptem, placenta of. 353
Chest wnll, of chick, iSi
Chorion: of hen's egg, 47; of

mammal, trne and false, 348 ;

ot rabbit, true and false, 353 ;

of hoinan ovam, 35J— 358;

of dog, 358
Chorion liEve, 356 — 358
Chorion frondoBQm. 3;Q — 358
Chorionic villi of manunal, 349
Choroid onat of eye, of cbicli:,

Choroid plei

s of mammalia.

Clu^imio acid. 417—498
Cioatricula, 4

Ciliary ; gangUon of chick, iiS ;
ndgea of ohiok, 141 ; miudeB,

EX. 475

Circnlation : in chick of saoond
day, 105; of third day, 110 —
tt3l of chick, later stages,

Oirculatory system of chiek, r6-

Borafi, 198—303
Clavicle: man. 4051 ofchiok.134
Cliuotd ridge. posIeHor, ohick,

308
Cock, con i- vase uloei, parepidi-

dyniia and vas deferens of, 114
Columella of chick. 166, 145
Commissnies of spinal cord, 153,

Coni-vasonlosi of cock, 114
Cornea of chick, igo — 153; of

mammalia. 39a
Comu ammonia, [ite Hippoc.

major)
Coracoid of ohick, 134
Coronal? vein, mammalia, 409^

Corpora bigemina of chick, 111
Corpora mBmmi)aria, 378
Corpora cjundriKemina of mam>

malia. 370: genicnlata, 371
Corpuij albicans, 373
Corpus caUoBum: mammalia, 381;

roBtnun of. 383; of marnnpials,

383 : of raonolremo^, 383
Corpus luteum, 311
CorjiuB striatum, mammalia. 378
Corrosive sublimnle, how to lise.

416
Cotyledonaiy placenta, deriratiun

of, 364
Cotyledons, 359
Cranial fleiore: ofchiek. ri6, 196:

of secood day, 101 ; of rabbit,

33^ ; of human embryo, 338
Cranialoerves : of chick, 113— iiy,

103 ; of second day, 101 ; di>-

voloproent of, 117 — i»g; of

mammalia, 400
Cranium of chick, 13; — 14> i

Cranium, mammBliu, 401
Crora cerebri, 37 1
Crypts of placenta, 360 — 363
CamuluB proligerue, 310
Cupola, .197, 398

Decidna: ofbumau placenta, J56;

nflexa in bumwi, jjfi — 3.sB ;

™™i 3S6— 3SB i aeroUua, ajfi—

3j8 ; reflem in dog, 359
Deoiduate plaoenla, 351 ; liialolot;y

of, 360
Dentary bones, 146
Dentine, loaminalia, 411
Dbiormbt's membrane, chiok. 1 5 ■
Diaphragin, muscleB of. 31 1 :

mamoiaba, 406
DiHaHB placenta, 359 ; hiBtoloio*

Diaooidal placenta, 353

Dog, plaoenta of, relation with

plawnta o( rabbit, 358
Dorsal aorta of chiak, 1G7
DnctuH arteiioHUs, man, 408
Dnctu? cDchlearis of ebiok. 159
DuctQs Botalli of chick, 387, iHiy,

396; of mamnialla, 40^
DnotuH Cuvieri of chicK, 170, n8,

184
DuotUH vcnoeus of chiuk, [6g,ii'i^

of mammalia, 413
Duodenum of chick, 171 — 174

Ear: of chick, 156 — 161 ; of mam-
malia, 39°— 397 ; accessor)-
■traotures of, 397 — 399
Egg tabes of Ffltlger, iii
Egg membranes of mammal, 310
Egg, to open, 437, 43B
Elephas. placenta of, 358
Embryo of chick : direotbns (or
examining, 4S9— 4S9 : "' J'*—
48 hours. 4J7— 4^*; oi 1,% W
50 hours, 444— 4A7 ; ot w>i^a

Jay, 447 — 451 ; o( fonrth day,
4J1— 4S3; of 10 houra, 453^
456 ; before incubation, 4.';7 ;
segmentation, 418; blood-i-es-
sels of, 459
Embryo of mammaU; direclionB
for eiamination of, 461 — 470 ;
o( segmenting ova, i — 7ihatirB,
461 — 4G4; of blBBtodermio veai-
ole of, 71 — 90 hoiira, 465 ; of 7
days, 465 ; of 8 days, 466 j of
a daya 11 hours, 468 ; of 14
days, 4^0 : of fcotal mom-
branen, 469
Embryonic area of rabbit, .117;

oompoHition of, 3 1 7
ICmbryonic membcimea: in mam-
malia, idpal type, 343 — 353 ;
yolk sao of, 34S— 35' ; amnion
of.34i— 3S1 : aUantois o^ 34s—
35 t; zona radista of, 345; se-
rous membrane 01, 345 ; oho-
rioa of, 34; : shedding of, ftt
birth, 3gi ; monotreraata, 353;
marsupialia, 351: rodeatia,
553. 3S4i inseetivora. 353;
cbBrioptoro, 353 ; man and
npe^.^Sf— 358; oarmTO»,358i
hjTai, 35S 1 elephas, 358 ; 01^
CeropuB, 358, horse, 359; ing,
35y; lemurs, 359
Embryonic sac in ehiPk, 57 — 38
Embryonic shield of chiek, 49,

."—5*
Enamel, 411

Endclymph. mammalia, 396
Epiblast ; formation of, in idliak.
7i, lA; derivation of, 36; c^
rabbit embryo, 316; hiBtologioat
differentiation of, in ehiek, 17 1 ;
opidonnis,37i inerToussyBtem,.
37 1 ; aense organs, »j» ; montb.
373 ; anus, »73; pituitary body.
373; salivary glands, 373; of
blaHtoderm from 8(h lu nth

Epididymis, mammalia, 4i(

Epiotic of chick, 346

Epithelioid lining of heart of'

Y.^AWivimsft^'oisiW of chick, iB»

^^^^^^^■^^H

IMSEX. 477 ^

Epoophoron, of hen, 114

Fonmen ovale : of heart of chiok.

Etbmoid: region, chiok. 340;

363. 364, 389, 397, 301
Foramen of Monho, 371

lateral, 141 ; bone, rhick, 146

Forebrain: ofchiok.ioo; of rah-

bit, 339: of mammalia, 371—

Eiistachtrin valve: of heftrt of

cbick. jfij— 4

Extemnl auililory meatus of mftm-

385; olfactory lobes, 38}

malia. 398

Foroxut of chick, formation of,

Extornnl carotid artPTj, uMck, 13;

81—8.

Formation of the layers in mam-

mals, 314—315

387-- 390

Formative cellfl, 33-3+

Eyelids, of rhJck. 155: of mani-

of. 383

Fonrth venCridc, chick, iii ;

mammalia, 368

Fourth nerve, chiok, iiS

t'ooe of ohiok, 146; nf hitman

Fretum Hallcri. chick. 139

embryo, J40

IVontal bones, chick, 346
Fronto nasal process, chick, ifi:.

Faciil nerve (.« Sovuntb)

303. u6

Fallopian tabes, mammalia, 415

False amnion of chick, 4^

(iaU-blodder of chick, tSi

Fiuoionli lereten, 368

Oasaerian ganglion, chick, 138

Female pronocletu, 17

ai4: of mammalia, 414—415

Femnr, chick, 134
FenaBtraovaUs, of chick, 166,545;

Oenerative organs, external, mam-

malia, 415—417

Fenestra rotnuaa of ohiek, 166,

Oenital ri<lgc, chick, 110

Giirm cells. pHmitive, of chick.

Fibula, chick, 334

Fifth nerve o[ chiok, ufi— iii),

Germinal disc of chick, u

103

Germinal epithelium, 113

Fifth ventricle of man, 183

Oerminal layers of chick. a6

First cerebral vesicle of ohiok.

Germinal vesicle of chick. 1 1

Heeond day, 57

Germinal wall, 53 ; stmc-ture of.

FissareB of spinal cord, 154

6fi — 66; fiinction of, 61

FIoccuU at cerebellnmof birdB,3ng

Glomeruli of kidney of chick.

580: amnion. 276— 378; allan-

toifl, j;7; yolk-sac, 377; mem-

366 '

branes of mammal, to examine.

Glomerulns of Wolffian body of

Folduig-off of embryo cWck, 1 1 3,

chick, 191
Glossopharyngeal nerve('fe Ninth

Gold chloride, ^

FolKcle, ovarian, n— is

*78 INI

GraaGtui follicle, chick. 121, tio
Grey matter, uf apiuol cord of
chick. 153; of brain of mam-
malia. ^187
Growth of embiyo of chick. 70
Ouinea-pig, Btiiictnre of UbbIo-
<lerm of, 333; relatign of cm-
brjonio layers of, 313 ; inver-
sion of the layers in, 341

H
Hsniato^iyliii, tu make and use,
45g

Bardemng reagents, 4]£ — 438 ;
picria acid, 41$ : carroaivc sub-
limate, 431 J OBmio avid ; 417 ;
ohromio acid, 417 ; ibiiotatQ
alcohol, 418 ; [he n«oessity of.
438

Head of chick, loo ; of rabbit,

Heart of diick. 310 — 330, 156 —
164 1 formation ol, 83— -89, 103;
beating of, on second day, 89 ;
of third day, 167; anricleB,
35(1 — 361 i venlriclos, j6o — 361 ;
Btuioular aeptiun, 357 — 161 ;
veDtrieularaeptnm,3 57 ; canalia
TenntenB, 157— 'SOi bnlbus ar-
terioguB, 3S7 — "M ; fommen
ovale, 3fi)— 364 ; Enstaohian
valve, 363—364; oiroolation in,
363—364; itructote of, 387 —
1B9, 393- — 1(|7 ; rCsumfi of, 39^

Reait of mammals, 339; alruc-
tnreof,^3i; formation of. 406;
comrariBon o[, with birdi^. 407
BemiazjgoBveiu. mammalia, 41]
Hen: formation of albumeu in,
16 ; o\-atiiin foUicIo of, 11—15 i
mesovarium of, 11; ovary of,

Hen'B egg, albmnen
blastoderm, 7 — 10, lO,
chalazffi, 4 ; oicatriaula, 4 ;
piegnation of, 17; laying
17; polar Lodiea of, 17; t^
mentation of, 18—34; vitellina

membtanc 0:

of, 4

-7; o

47 :«

rregnlar develc^

ment of, 48, 49 ;

oftvity of, 50
Heiiatio oylinders of ohick,

oircolation of chick, 3 1 7 ; rein*,

388—1510
Hind brain: of ohick, 100 ; of

rabbit, 339 ; of mammals, and

birdB, jfij—ijo; niednlla of,

367; cerebellum of, 36;— 370
HipjK>-Dampua major, mammalia

380 ^

Hippo-campal fiseure of oersbnun

of manniiaiia, 385
Histological differentiation,

chick, 369—373; of epiblaat,

169, 171; of hypoblast, 369,

of mesoblast, 169
Histology of placenta, 359
Holoblastio eegtnentation, 307
Human emliryo : villi of, 331

340 ; mudallory plate of, jj-f ,
amnion of, 338—340; oroniai
fleiuroof, 338—340; limbs oT.
339 ; body (ieiura o^ 339 —
3^0; face of, 340; relatimi ot
with other mammals, 341 ; pla-.
ceotaot, 3SS
Human ovnm, eise of, 307
Haman placenta, bistolog]

363 ; derivation of, 364
Humenia. chick. 3.14
Hyaloid membrane, r^iek.

BpoophorOD, of hua, 1 14
Ethmoid: isgion, chich, 140;

latonl. 14! ; bone, chick, 146
Enstaabiui tube: of chick, 1^5;

of rabbit. 334; of niiimmalia,

397—418
Enstashian valve ; of heart of

External auditor; meatUM of mam-

mmlJB, j,gH
Eztemal carotid nrlery, chick, iig
Eye; of chick, loo; development

of, 131 — 15; ; of niammnliui

387—39*
EyelidB, of chick. 1551 of mam-

malia. 390

Face of chick, 146; of human

embryo, _i40
Facial nerve («« Seventh)
F&ldform liitainent, mammnliu,

Fnllopisn tubes, niatomalia, 41;
Fttlse amnion of chick, 46
Fall cerebri matninalia, 377
Fasciculi tcrcten, 36 S
Peathen, formation of. iHi
Female pronocleua, 1;
Femur, chick. 134
Fenestra ovaliR, ill chick, iM, 14;;

103

Fifth ventricle of man, 383
First cerebral vcidcle of chick,

second day, 97
FiBnnras of gpinal cord, 754
FlocoiiliofoerGbelluinofbirdB.369
Fietal appendages : of chick, 17^—
180; amnion, ,76—178; allan.
lois, i77;yDlk-aac, 177: mem-
branes of mammal, to eiamiQe,

Folduie-otf of embryo chick, 1 13.

FfJiioIe, o'

EX. i77

Foraroeu ovale: of heart of chick,
ifii, 164, 389, 297, 303

Foramen of Monbo, 371

Fore brain : of chick, 1 00 ; of ral)-
bit, 339; of uiomnialia, 371 —
.585; opticveHicleBof,387— 390;
thalamencephalon, 371 — 376 ;
cerebral hemispheres, 37O —
385: olfactory lobea, 38s

Foregut of chick, formation of,

Formation of the layers in roam-
mal». 314— 3JJ

Fornix, mammalia, 381 ; pillars

of. 183
Fonrth ventriclt, chick, 11:;

mammalia, 368
Fourth nerve, chick. 118
Fretum Halleri. chick, 119
Frontal bones, chick. 346
Fronto nasnl process, chick, 16;,

10,, 1+6

Gall-bladder of chick, tSi

Gasserian ganglion, ohiok, iiS

Gonerativeglanila : of chick, no — -
314: of mammalin, 414^415

Generative organs, external, mam-
malia, 41s— 4' 7

Genilal corI, mammaha, 415

Qonital ridge, chick, no

Germ cells, primitive, of chick.

Germinal disc of chick, 11
Germinal epithelium, 113
Genmnal layem of chick, iG
Germinal vesicle of chick, 1 1
Germinal wall, ^1 ; stracture ofi

65—66; fnnc^on of, 66
Oloniemli of kidney of ohiok,

Glands, epidermic, of mammalia,

366
OlomeniluH of WolCBan body o(

chiok, 191
GlossopharjugealncrTe("<Kinth

Gold ebloride, 460

r ^taim

w -.

480 INDEX. f

Limbs, of ohick. 198 — 100. i j j ;

MeduUarj groove, of ohiok, 19,

at rabbit, 334 1 of hamm em-

61 — 6;; of rabbit, 310, 311;,

L^v&rtr\tir^.m.

ot mao, 338; cIosorB of, in

mammal, a^J— 33'
MednUary pUte. ot chick. 61 ; of

malil. +,^

LmnbftT veinii. mamnialia. 41J—

rabbit, 310; of man, 338
MembraOB oapHuIo pupiUaria ot

LangB of cliick, 176— 1;8, ifi? ;

mammalia, 387—389

Membrana limitana externa, 145:

■

Membiana propria ot folliclea.

■

chick, iSi

■

Membrane : of shell of hen's eag.

, ; serooB, of chick, 51-4. ;

^ Mala prunocleuB, 17

vitellioe of ben's egg, 13—15

MtUUiiB. 398, 404

Membrane bonea, 141 ; of skoll.

MalpighiiiQ eorpuBolcs, chiok. 1 81 ;

chick, 146

bodieBof chiuk, 190

Mwninalia, two periods of davelop-

lia. 397

meut. 308 ; viviparons, 308

Mammary glandB, j66; a maioe

of nntriment for the embryo,

uiammolia. joj
Membranous labyrinth, chick.

308

Man («* Hnman erobijo)

158

Meniacna of birds, no

144; maiillarf process of.

ohidt, 343: rabbit, 3J4; mam-

388—190

Manubrium of mallens, 403

mnlia, 419—10
MeHobla«t:deri7fttivcaof.in chick,

iS^

IS— 16; of primitive streak of

cbick, J4, si; derived from

Matnration of ovum of mammal.

lower layer cells in chiok, 55,
57, 59 ; of area opaca in Dhiok,

310

Maxilla bones, ohick, 146

fiSispUttingof.iu chick. 68; rf

trunk of embryo ohick, i8s —

lar arch of cbick, 143

of, in chick, 169: of primitiTB

Meatua auditurins eiteruui, of

streak of rabbit, 310; of mam-

chiok, 166 ; ot monimal, 397

loal, double origin of, 311 —

Meatas reoosus, of chick, [69,

3J3; Tcrtebral zone of, 318;
lateral zone of, 318; somite*

iM?

of, 318

mamniolia, 403

inchick, 7oiotchick,8., 185—

of luainmaliH, 367

187, .o^-a^ '

Medullary canal, of chick, 40, Gi,

96
Medullary folds, of chick, 40, 61,

MeBocac.linm of chick, 88; fornm-
tion of, 164

■k

-iM

MaaoTUiiini of fowl. 1 1

UetseuptiB, chick, 334
Uetadiscoidftl pUeenta. hiatologjr

of, 361; deriTBtion of, 364
Metamorphosis of arterial arohea.

bird and mammaliB, 408
Metanepbos |irr Kidney)
Metanepliric blHBlema. of chicly

119
Uiciotomes, and makers of, 434

— 43S: 47'
&tid fanun: of chick, 100, loo; of

rabbit, 319; of mammalia. 370

371 ; ventricle of, 370; nates

and teatea of, 371: corpora

genieulaU, and crura cerebri of,

371
HoDoti«inata, foetal menbraDes

ot. 35»
House, tnTersion of the layers in,

Nails, of chick, 18]

KaroB : posterior, chick, 351 ;

torior and posterior, of n

malia, 399
Nasal capsule, chick, '

Kales of mammalia, 3; 1
F.AB.

mole, 316, 1

Mouth, chick, 349, 1811 of rabbit.

formation ot, 33^
Miilleriau duct : chick, 114^118;

mammalia. 4 1 4—4 1 5
Muscle plates (if chick. 187— 1B9,

104 — 108, 111; aegmentation

of, 111

Muscles; hypoakeletal, chick, 111 ;
episkelet^ chick, 111; oata-
nooUB.ahick.ii I ; extrinsic and
intrinsic of litnb, chick, tii

Muacolar wsJIb of heart of chick.

Nerres, of chick of oeMiiid iaj,

lOL ; of itreiTTiDfllia, 400

Kervous system ot mammalia,
367—400

Neural band, chick, 113; crest,
116

Neural canal of chick, 31 — 39,66;
second and third day, in; de-
velopment of, 151^556

; canal, ot chick, 71 —
" . 399: 0*
■. 3>«
>, chick, i]G — 119, 30J

Node of Hansen. 319

Non-deciduate placenta, .tsi

Nose, chick, 149

Nostrils, chick, iji

Nolochord: of cluok, 19, 6a — Gi,
ao8 — 110, 137—538; ot second
day, 101: sheath of chick, loS;
of mammal, 313, 400; forma-
tion of. 31s

Kuclei, iG

Kuoleotoa, tj

Nucleus, 13

Occipital : snpra-, basi-, ei-, of

chick, 34G; foramen, chick, 137
{Esophagus ot chick, 173: mam-
Olfactory orsan of chick. 161 ;
nerve of diick, 161 : grooves,
chick, 101 : lubes of mammalia,

Obvnry bodies, 36S

Omenlam, mammalia, lesMr,4io]

Hireatrr, 410
Opisthotic ot chick, 146
Optic vesicles : of chick of seoond

d»?. 79. 97: oh'"''. 133— I J4!

formation ol, 141 — 14^1 at

rabbit, 319

482 INI

Optic lobes, shidi, iii
Optia narves. chick, 133, 146
Optic cup, 134

Optio chmsm&, chick, ht, mam-
Optic Calami of mammalia, 373
Orbitospbeaoid, 14G
OrbitoaphsDoidiJ region, chick.

399
Oryoteropufl, placents of, 358
Osmio acid, how to use, 41;
Oaseoaa lab.Yiioth, chick, 158
Otic vesicle, chicli, 1 j 7
Outer la;er, ot blastodermic vet

ole, 314
Ora, primordiBl, of chick, 11 1
OTarian follicle: ot ben, 11— 11

mammal, 309
" — '"" : of hen, 11— i;

ot mam: . _,
Ovaij: of adult hen, 11 ; of

chiok, 111 ; of mammalB,

V>9'- follicles of, 309; corpus

Inteiim of, 311.
Ovidnct of ndalt hea, : j ; of

chick, J14
Oviparous animals, 30S
Ovum : of birds and mammals

compared, 307 ; of mammal —

in follicle, 309 : membranes of,
' ■ "' nd imprag-

of.

Palate, mammaUa, 41a, 411
Palatine bones, chick, 346
Pancreas: of chick, igi ; mom

malia, 4IU
Paoder, nucleus of, 7.
Parachordals, chick. )3i^i38

POok.

1I4

Parieto-oocipital fiaran of 1
brum of man and apea, 38;

Pareeh on the fowl's efctul, 145

Paroophoron of hoD. m

Peoten, chick; 147

Peotoral girdle, ^ick, 334; mam-
malia, 40=.

Pelvic girdle, chick, 134 ; mam-
malia. 405

PcDiB, mammalia, 417

Pericardial cavity, ^ck, devdop-
ment of, 164 — 169 ; of nblnt,
331 ; mammalia, 406

Fenlymph. mammalia, 396

Periotio capsules, ohick, 137

Peritoneal covering of beatt of

chick. 8S; Cavi^, ni»rHTn«li»

■ 406
Peritoneom, mammalia, 419 — 410
pTLaoKB, egg tabes, m
Phalanges, duck, 134
Pharjnx, mammalia, 41S
Picric aoid. bow to Dse, 41;
Pioro-coimine, to make and lue.

Pig, placenta, hiatology at. 360

Pineal glaoda, chick. 117 — 115

of mammalia and birds, 373-

Pitaitaiybody; chick, iig — t
rabbit, 334 ; ot birds.

1, 37»i.4»»

19)

.17»;

[, 140

bodies of, 311 ; scgmentatioi
o(, 31]— 314; blastopore ot
(BenedeD), 314

leddua of, 356 ; ohoriOD W™
of. 356— isS; chorion (1 '
sum of, 356 — 3j8 ; coup
of. 358; loaaij type ot, _
diffuse form, 359; poljco^la-
donary form, 359; hialologyirf.
359—3^3! evolutioD of, 364:
of sloth, 360.

Pleural cavity, cbiok, development
of, 164^ — 76g i nmntmalia, 406

Pleoroperitoneal spac« of chick,
^8-^3, 84; tonnation of, 40, j

FneumoiiaBtrio nerve (trc Tmlfc I

FoWl

-, bodiee, 17 ; of otb of uam-

maU.3(i
Poljcotyladonftiy placenta, 359 ;

hislology of, 360
Pods Varolii of birds, 369 j of

msmmalB, 370
Poaition of embryo oMclt of third

BDd fourth da>s, 1 13 — 1 16
Poataniil gut, of chiok, [jj; at

rabbit, relation of, to primitive

streak, ^ig
Posterior narea, ohiek, 301
Potaasiam biohromate, 460
Preraaiilla bones, chick, J46
Plenaaol banes of chick, 346
Presphenoid region, ohiok, 340—

346
Primitive grooTC of chick, 56 ; of

rabbit, J JO
Primitive streak of ohiok, sj — 61;

of chick from 10 to 34 houra.

70; of rabbit, 319
Pn>ce«aaa infimdibnli, chiak, 111
Proctodnnim of chiok, 175; of

mammal, 431
Pronephros, 318

ProBOoieua, female, 17; male, 17
Prootio, chiok, J46
Protovertebtffi [tee SleBoblaatio

Pterygo-niiUtine bar, chick, 343
Pler^ygoid bones, ohiok, 346
Pubis, chick, 134
Pulmonary veins of chiok, 138,

389—390
Pulmonary orteries of chick, 394 —

308; mammalia, 407
Pupil, chick. 143
P/ramida of cerebeliuni, 36B

Qnadtnto-iugal boiies. :
Qnadrote, ohiok, 343

Rabbit ombtjo, growth of, 337-
334; placenta of, 353
a», ohiok, 134

EX. 488

Bat, inveraioii of the layera in,

Becesaua labyrintlii, mamm ,,

390—39'^
BeocBBns veatibuli {tcf Aqnednctul

veatibuli) chick, 303
fiespiration of chick, 303; oftblil

dny, no
Rete vaaculoaam, mammalia. 414
Retina, chick, [43, 144— r|(S
Ribs, chick, 3341 manmialia, 409
Bodcntia. phiMnla of, ^J3
Rods and oonea of retina, chiok,

(46
Itostrum, chick, 146
Kuminanta' placenta, histology of,

Sacculns hemiBpharicuB, mam*

nialia, 390 — 398
Salivary glands, mammalia, 430
Scala meilia \iee Cochlear canal)
Soala tympsni, mammalia, 39J —

397
Seala vcBtibuli, mommalia. 395 —

397
Scapula of chick, 134
iialcrotic ooat of eye of chiok, 141
Sclerotic capsalea, mammalia. 403
Sdolum, mammalia, 416
SeboceouB glands, 366
Secondary optio veaiole (tte OpUe

oup)
Sectioni, method al cutting, 434

— 436; mounting of, 436
Segmentation : of hen 'a egg, iS

Hegmentatioi
— 14^ raeroblaatiu, iH

hen's egff to observe.

-3M

mammalian

Suuiicinmiar canal : of chick,

1S8; mammalia, 390—398
Semi-lunar valves, cUck, 3j8
UenBo capaulea ol chick, 111 — in
Heptum lucidura, mammalia, 38J
Septniu-nasi, chick, 346
Serona membrane of chiok, 3*—

Saroua entelope of ohidi,

Sheh-membnute of chiok, i

Shell of lisn'a egg, i ; formation
of, i6

Shield, embryonic, of eliiek, 49

BinaB chomboidalis : of embryo
ohick, ;i, 81 ; of rabbit, iig

Sinna toimiiialis, of chick of
eeoond day, gi, 104-, in rabbit,
343

SinuB venoana of ahick, 169, 216,
185—190

Skeleton of limb, chick, 134

Skull of ohick, ij; — 151; cartilage
and membrane bones of, 146 ;
of mammalia, 401^405

Bloth, placenta, liiatology of, jfio

Somatic stalk of ohick. 19 — 41 ;
of mammals, 35 1

Somatoploure of ohick, 19 — 33;
tonnation of, 40— 41, 68

Spermatozoa of chick, 113

Spioal nerves : of ohick, 113; ds-
yelopniBot of, 119— 131 ; of
mammalia, 40a

Spinal cord of chick: develop-
ment of. jsi^is^ ; white mat-
ter ol, iji; grey matter of,
153; canal of, ssi— ]j6 ; epj-
thelinm of, 151, 151; anterior
gre^ commiBBHre of, «56 ; an-
terior fissure of, 354 — 256;
dorsal fiasute ol, 355^156 ;
posterior grej commiaBure of,
1 j6 ; sinua rhomboidnlii ol,
256; anterior columns of, 156;
posterior oolamna of, 356 ;
lateral columns of, 1561 an-
terior white oommiaaore of,
ij6; posterior white commis-
sure of, jj6

Splanuhnio stalk of chick, 19 —
*a. J3'

SplancUnopleurn of ohick, 19—
13 ; formation of, 40—^1, 68

Spleen of chick, 18a
■Uot bones of chick, 146
I inosal bones of chick, 346

ining leaeenta, 418 — 431: h*-
natoxylin, 4i9;l>orai carminei

398, 404
Stemiim ol chick, ta ; of

Stomach of i

'73;

Stomodieum, of chick, 119, 103*.

mammalia, 410
Stria vaBCUlaris, inammaliA, 39;
Subclavian nrtories of chiok, 196

— 198, of mammalia, 409
Sabclavian rdns, mammalu, 409

Suloua of Monro. 373

Superior maiiUa of ohick. i6{ ;

maiUlary proc^Bsea of chick,

101 ; of rabbit, 334
Superior cardinal Teins of ehkk,

118
Supra-renal bodies, mamiualia,

Btracture of, 413; reUtion of,

with sympathetio nervom sy»-

Subzonal membrane of mammal,

346
Sylvian Qssnre, mammalia, 384.

385
Sympathetio nervous syatem of

mjtmmalia, 400
Sweat-gland", 366

Tail -swelling of ohick, 74

TarenB of chick, 334

Teeth, mammalia, 411

Tela cboroidea, 37^

Tenth nerve of chick, iij, 117 —

up, 103
TeatiB of chiok, jii. 371
Thalamencephalon : of chiok,

117; of mammalia. 371 — 376;

ventricleof, 371; &oorot,'};x,

373! iidMof, 373i roof of, 374

Third nerva of chick, ng

Third Tentricle of manmiaUa, ,^73

Throat of rabbit, forniBtion of,

3J'
Thyroid body, of ohick. 181 ;

mamma] ia, 41S
Tibiaofchiok, ]j4
Tongue of ohick, 381
Tra))eaiiln of ohick, 136, 131J — 14I
Trachea of ohiok, 176, 177 ;n]aiii-

Taber oinerenm, .;7i
Turbinal bonea of oftiok, 146
Tpnpanie cavity of chick, 166 1
membraDB of chick, 166; cavitj
of mammaUa. 397, 418-, mem-
brane of maonnalia, 397

rioft. of chitk, 134

Umbiliml. arterieii [fte A11«ntuic):
veina [iff AUaDtoiovsinskvaii-
cle uf mamniBls {let Yolk-Hac) ;
xtklk of chick of third d«]r, 113;

lIrBohtlB,'3ji

Urettr of chick, Tig; mammaliH.

Urethra, niammalia, 41;

Crinogenital oritana uf iiiaiu-
malia, 414^41 71 ainue of mam-
malia, 415 — 417

Uterine orypts, 310

Uteran, mammBiiia, 41 j

UlriculaH of mammalia, 353 — 398

Uvea of iris, chiok, 1 44

TBlveofVieuuena, of birds, 369;

of mammaU, ^70
Vagina manunalts, 41;
Vague nerve {ift Tenth nerve)
Vana eflerentia and recta mam-

VoBCular nyBtem of chick, 114 —

— 106; of third da;, 167 — 170;

mammalia. 406— 413
Vanoobir area: of blastoderm of

chick, 37; of third day, 110—

[ 13 1 of rabbit's ovrun. toima'

tion of, 316
Vas deferens : of nock, 114 ; mua-

Vermiform appendix, n

t'i

, 409

Vuoffi odvehentes of chick, 117,
387^989; revelientei otchieki
1)7, 187—189

V'eiift termiualJH I'te Sinua lemu-

Venous iiy»tem; of cbitk, ji6--
5ig, 1B3— 190,301— jo3;mam-
malia. 409— 413

Ventricles of brain of eldA of
second day, 101; of

1]; of at

Ventricular septam, chick, 1301

Vertebral artery of ohick, igs —
198

Vertebral colnmn, of chiek, loj —
loS ; membranous, 105 — 108:
secondary segmentation of, loj
— 108 ; explanation of do,, loj
— 106 ; of mammalia, early de-
velopment, oHsificatioa of, 400,
401

Vertebrate nnimal, genera! slrae-

486

INDEX.

Yesaels of placenta, 360—563

Vestibule, chick, 158

Villi: of human ovum, 345; of
zona in dog, 347; of subzonal
membrane of rabbit, 347; of
chorion of mammal, 349; of
placenta, j6o- 363

Visceral arches, 345 ; of rabbit,

334
Visceral arches of chick, i6a — 167;

of rabbit, 334; of mammalia,

407
Visceral clefts: of chick, 162 —

167, 981; closure of do., 164;

of rabbit* 334; of mammalia,

402, |i8
Visceral folds of chick, 163
Visceral skeleton of chick, 343

^ — 246
Visoeial vein of chick, 284 — 290 ;

of mammalia, 400 — 413
Vitellin, 5
Vitelline arteries: of chick, 167,

393 — 298, 225; of second day,

89, 103
Vitelline duct of chick, 196, 232 ;

of mammals, 350
Vitelline membrane, 4; of hen's

®K8f '3— »5; of mammal, 310
ViteUiue veins of chick, 84, 226,

288 — 290 ; of second day, 92,

104; in rabbit, 343; of mam-
malia, 410 — 413
Vitreous humour of chick, 140, 1 50

Viviparous animala, 308
Vomer of ohiok, 146

White matter : of spnal cord of

chick, 251; of brain of mam-

maliaj 386—387
Wings of chick, 100
Wolffian body: of chick, 190 —

193; of mammalia, 414; of

cnick of second day, 106
Wolffian duct of chick, 190, 113 ;

of second day, 94 — 95, 106; of

mammalia, 414
Wolffian ridge of chick, 198
Wolffian tubules of chick, 106,

191— 193, «i3

Yolk of hen's egg, 4—7 ; arrange-
ment of, 6; structure of, 5

Yolk-sac: of chick, 28 — 37, 277 —
280; of mammals, ^27; of
marsupials, 352; of rabbit, 353;
of human ovum, 355 — 358;* of
dog, 358

Zona radiata, 310; of chick, 15
Zonary placenta: histology of,
360 ; derivation of, 364

CAMBBIDQB : PRniTBD Wl C. J. Cl^T^^SL-K. ^^^, ktT\Lia.\i^\S'B»»v-n vuxaa.

^^«5;

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